Image forming apparatus having reduced image errors from image bearing bodies and method of manufacturing same

ABSTRACT

An image forming apparatus has a plurality of image bearing bodies in a predetermined sequence, an intermediate transfer medium for running in contact with each of the plurality of image bearing bodies, and a plurality of supporting rollers for driving the intermediate transfer medium. At least two image bearing bodies are oriented such that radius deviations of the respective image bearing bodies are in substantial registration along the intermediate transfer medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No.2004-48837 filed Jun. 28, 2004, and Korean Patent Application No.2005-43735 filed May 24, 2005, in the Korean Intellectual PropertyOffice, the disclosures of which are hereby incorporated by reference.This application is related to U.S. Patent Applications titled “Rollerfor Image Forming Apparatus and Method of Manufacturing Same,” “ImageForming Apparatus Having Reduced Image Errors From Driving Unit andMethod of Manufacturing Same,” and “Image Forming Apparatus Adapted toOverlap Image Errors From Driving Unit and Image Bearing Bodies andMethod of Manufacturing Same,” each filed on even date herewith.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a driving unit for an image formingapparatus, a color image forming apparatus having the same, and a methodof manufacturing an image forming apparatus having the driving unit.

2. Description of the Related Art

Generally, image forming apparatuses are classified into a monochromaticimage forming apparatus and a color image forming apparatus. Themonochromatic image forming apparatus forms an image in black and whiteby using only one color developer, while the color image formingapparatus forms a colorful image by using color developers such asmagenta, cyan, yellow, and black.

An electrophotographic image forming apparatus forms an electrostaticlatent image by scanning a laser beam onto an organic photoconductor(OPC), which is electrified by an electrifying unit to have apredetermined level of electric potential, using a light exposing unit.The electrostatic latent image is developed using a developer and thentransferred into a visible image on supplied print paper. In a colorimage forming apparatus, the electrostatic latent image is developed byusing color developers on the organic photoconductors and overlappedimages are transferred onto an intermediate transfer medium, such as anIntermediate Transfer Belt (ITB). The color images overlapped on theintermediate transfer medium are transferred onto print paper.Subsequently, the print paper with a color image goes through a seriesof fixing processes before it is transferred out of the image formingapparatus.

FIG. 1 is a structural diagram showing a conventional color imageforming apparatus that performs a two-step transferring process by usingthe intermediate transfer medium. Referring to FIG. 1, the color imageforming apparatus comprises a belt 10, a supporting roller 11, four T1rollers 12, 13, 14 and 15, four organic photoconductors 16, 17, 18 and19, a T2 roller 20, and a belt driving roller 21.

Developers each corresponding to K (black), C (cyan), M (magenta), and Y(yellow) are attached to the electrostatic latent image area of each ofthe organic photoconductors 16, 17, 18 and 19. The T1 rollers 12, 13, 14and 15 are set up to correspond to the organic photoconductors 16, 17,18 and 19 with the belt 10 between the organic photoconductors 16, 17,18 and 19 and the T1 rollers 12, 13, 14 and 15. Accordingly, thedevelopers attached to the surfaces of the organic photoconductor 16,17, 18 and 19 are primarily transferred to the surface of the belt 10 bythe transferring activity of the T1 rollers 12, 13, 14 and 15. Herein,the respective color images of the organic photoconductors 16, 17, 18and 19 are transferred onto the belt 10 at predetermined time intervalsso that the color developers transferred to the belt 10 can beoverlapped in registration into a complete color image. Therefore, thecolor developers are overlapped on the belt 10 to thereby form acomplete color image. Subsequently, the color image on the belt 10 goesthrough a secondary transferring process between the T2 roller 20 andthe belt driving roller 21. Also, the belt driving roller 21 runs thebelt 10 at a proper rate.

The belt 10 and the organic photoconductors 16, 17, 18 and 19 areconsumption goods with limited life spans, that must periodically bereplaced.

A transferring unit including the belt 10 and developing units includingeach organic photoconductor 16, 17, 18 or 19 are typically set up in adetachable manner along with a predetermined coupling unit and thedriving unit for providing power in the image forming apparatus.

When the consumption goods, that is, the transferring unit and thedeveloping units, are replaced, the structure of the coupling unit forperforming relative motion of the driving unit, the transferring unit,and the developing units is significant. That is, it is important toconnect the transferring unit and the developing units to have the samerotational axis as the driving unit. It is also important to realizehighly precise color registration in consideration of the so-calledtotal runout between the transferring unit and the developing unit. Thisis particularly important in view of manufacturing tolerances in theexternal circumferential surface of the driving roller of thetransferring unit.

Herein, the total runout can be understood to include a phenomenon thatthe belt 10 rotates at a predetermined rate, but small changes in theinstantaneous rotation rate occur due to the manufacturing tolerance ofthe external circumferential surface of the driving roller. The totalrunout affects the developing units in the same period through the belt10. Therefore, it is important to equalize the influence of the totalrunout on the developing units and improve the quality of a color image.

Additionally, the organic photoconductors and the driving roller mayhave deviations in their radii if the outer circumferences thereof donot make a perfect circle due to manufacturing tolerances. Suchdeviations result in errors of a composite color image on the belt asthe color image partially stretches or cuts due to the aforementioneddeviations in the OPC rollers and the driving roller.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a rollerwhich enables controlling the orientation of a predetermined radialdisplacement.

It is another object of the present invention to provide a method ofmanufacturing a roller which enables controlling the orientation of apredetermined radial displacement.

It is another object of the present invention to provide an improveddriving unit of an image forming apparatus which enables minimizing animage error caused due to radial errors in rollers supporting the belt.

It is another object of the present invention to provide an improvedimage forming apparatus which enables minimizing an image error causeddue to a plurality of image bearing bodies.

It is another object of the present invention to provide an improvedimage forming apparatus which enables minimizing an image error causeddue to a plurality of image bearing bodies and belt supporting rollers.

The above aspects and other features of the present invention cansubstantially be achieved by providing a roller including a roller bodyhaving a radial displacement along the direction of its circumference;and a driven coupler engaged with one end of the roller body, forcomplimentary mating with a driving coupler for driving a driving force.The roller body has a mark formed on a predetermined position forindication of radial displacement.

The driven coupler includes a positioning part for determining aposition of engaging with the driving coupler. The roller body isengaged with the driven coupler such that the positioning part maintainsa predetermined angle with respect to the mark.

The mark is preferably provided at a predetermined angle with respect toa maximum radial displacement location of the roller body.

According to one aspect of the present invention, an image bearing bodyfor use in an image forming apparatus includes a drum body having aradial displacement along the direction of its circumference; and adriven coupler engaged with one end of the roller body, forcomplimentary mating with a driving coupler for driving a driving force.The drum body preferably has a mark formed on a predetermined positionfor indication of the location of maximum radial displacement.

The mark is preferably provided at a predetermined location with respectto a maximum radial displacement location.

The driven coupler preferably includes a positioning part fordetermining a position of engaging with the driving coupler. The drivencoupler may be engaged with the drum body such that the positioning partmaintains a predetermined angle with respect to the mark.

The driven coupler preferably includes a non-circular coupling partformed at one end either in concave or convex fashion to receive adriving force in an engagement with the driven coupler. The positioningpart may be extended from the coupling part in the direction of theradius of the driven coupler.

According to another aspect of the present invention, a method ofmanufacturing a roller including a roller body having a radialdisplacement along the direction of its circumference, and a drivencoupler engaged with one end of the roller body, is provided. The methodmay include the steps of identifying a location of radial displacementof the roller body; and setting the position of the driven coupler withrespect to the identified location and engaging the driven coupler withthe roller body in said position.

The step of finding the location having a radial displacement preferablyincludes the steps of finding a point of maximum radial displacement bymeasuring an end of the roller body; and forming a mark on the rollerbody for indication of the found point of maximum radial displacement.

The step of engaging the driven coupler preferably includes assemblingthe driven coupler with the roller body in a manner such that apositioning part of the driven coupler maintains a predetermined anglewith respect to the point of maximum radial displacement and that therelative positions of engaging the driven coupler and the drivingcoupler are determined.

The step of engaging the driven coupler preferably includes the steps ofsupporting the roller body on a first jig such that the point of maximumradial displacement is positioned at a predetermined angle with respectto a reference coordinate axis; supporting the driven coupler on asecond jig such that the driven coupler is positioned at a predeterminedangle with respect to the reference coordinate axis; and moving thefirst and the second jigs relative to each other and thereby engagingthe driven coupler with the roller body.

According to yet another aspect of the present invention, a driving unitof an image forming apparatus may include a driving gear for driving aplurality of image bearing bodies which are arranged in a predeterminedsequence, and a driving gear for driving a plurality of supportingrollers which support a belt running on the plurality of image bearingbodies. A distance between centers of rotation of at least two of theplurality of driving gears is preferably a positive integral multiple ofthe length of circumference of one of the supporting rollers.

Each of the distances between the centers of rotation of the pluralityof driving gears is preferably a positive integral multiple of thelength of the circumference of at least one of the supporting rollers.

One of the supporting rollers may be a driving roller which receives adriving force in engagement with the driving gear.

One of the supporting rollers may be a roller having a radialdisplacement.

The driving gear preferably includes first through fourth driving gearsin a predetermined sequence with respect to the direction of beltrunning, which satisfy at least one of the following equations:

[Equation Set 1]L 1=1·Sd·(1±0.05)(1=1, 2, 3, . . . )  (1)L 2=m·Sd·(1±0.05)(m=1, 2, 3, . . . )  (2)L 3=n·Sd·(1±0.05)(n=1, 2, 3, . . . )  (3)L 4=o·Sd·(1±0.05)(o=1, 2, 3, . . . )  (4)L 5=p·Sd·(1±0.05)(p=1, 2, 3, . . . )  (5)L 6=q·Sd·(1±0.05)(q=1, 2, 3, . . . )  (6)where

-   -   L1 is the distance between the centers of first and second        driving gears,    -   L2 is the distance between the centers of second and third        driving gears,    -   L3 is the distance between the centers of third and fourth        driving gears,    -   L4 is the distance between the centers of first and third        driving gears,    -   L5 is the distance between the centers of first and fourth        driving gears,    -   L6 is the distance between the centers of second and fourth        driving gears, and    -   Sd is the length of the circumference of the one of the        supporting rollers.

The driving gears are preferably mounted to satisfy all of the equationsin Equation Set 1.

The driving gears are preferably mounted to satisfy L1=L2=L3.

The driving gears are preferably mounted to satisfy L1=L2=L3, and L1,L2, and L3 are positive integer multiples of Sd.

The radius of the supporting rollers is preferably equal to the radiusof the image bearing bodies.

A plurality of driving couplers are preferably provided to a center ofrotation of each of the plurality of driving gears, to transmit adriving force in engagement with each of the plurality of image bearingbodies.

The driving coupler may include a coupling part configured to have anon-circular section corresponding to each of the driven couplersprovided to an end of each image bearing body; and a positioning partprovided to a side of the coupling part to determine a position ofengagement such that the driving coupler engages with the driven couplerat a fixed posture.

The coupling part may be a coupling recess sunken into an end of thecoupling part in a non-circular shape, and the positioning part may be arecess sunken into an inner side of the coupling part to a predetermineddepth.

The driven coupler may include a shaft protruding from an end toward thecenter of rotation, and a shaft hole at a bottom of the coupling recessfor engagement with the shaft.

The coupling part may be a coupling protrusion extending from an end ina non-circular shape, and the positioning part may be a protrusionextending from an outer portion of the coupling protrusion.

According to yet another aspect of the present invention, an imageforming apparatus preferably includes a plurality of image bearingbodies arranged in a predetermined sequence; an intermediate transfermedium running in contact with each of the plurality of image bearingbodies; and a plurality of supporting rollers for driving theintermediate transfer medium. A distance between centers of rotation ofat least two image bearing bodies is a positive integral multiple of thelength of the circumference of at least one of the supporting rollers.

The image bearing bodies preferably include first to fourth imagebearing bodies which are arranged in a predetermined sequence withreference to the direction in which the intermediate transfer mediumruns, and the first to fourth image bearing bodies preferably satisfy atleast one of the following equations:

[Equation Set 1]L 1=1·Sd·(1±0.05)(1=1, 2, 3, . . . )  (1)L 2=m·Sd·(1±0.05)(m=1, 2, 3, . . . )  (2)L 3=n·Sd·(1±0.05)(n=1, 2, 3, . . . )  (3)L 4=o·Sd·(1±0.05)(o=1, 2, 3, . . . )  (4)L 5=p·Sd·(1±0.05)(p=1, 2, 3, . . . )  (5)L 6=q·Sd·(1±0.05)(q=1, 2, 3, . . . )  (6)where

-   -   L1 is the distance between the centers of first and second        driving gears,    -   L2 is the distance between the centers of second and third        driving gears,    -   L3 is the distance between the centers of third and fourth        driving gears,    -   L4 is the distance between the centers of first and third        driving gears,    -   L5 is the distance between the centers of first and fourth        driving gears,    -   L6 is the distance between the centers of second and fourth        driving gears, and    -   Sd is the length of circumference of the one of the supporting        rollers.

The image bearing bodies are preferably mounted to satisfy all of theequations in Equation Set 1.

The image bearing bodies are preferably mounted to satisfy L1=L2=L3.

The image bearing bodies are preferably mounted to satisfy L1=L2=L3, andL1, L2 and L3 are positive integer multiples of Sd.

The plurality of supporting rollers preferably include a driving rollerfor driving the intermediate transfer medium while being rotated with adriving force transmitted thereto, and an idle roller beingpassive-rotated while supporting the intermediate transfer medium. Sddefines the length of circumference of a supporting roller.

A driving unit is preferably provided for driving the image bearingbodies and the supporting rollers.

The driving unit preferably includes a first driving unit for drivingthe plurality of image bearing bodies at the same time; and a seconddriving unit for driving one of the plurality of supporting rollersindependently.

The first driving unit preferably includes a plurality of driving gearsprovided to correspond to each of the plurality of image bearing bodies,and rotated in association with each other; and a first driving motorfor providing a driving force to drive the plurality of driving gears atthe same time.

The second driving unit preferably includes a second driving motor; andone driving gear engaged with one of the supporting rollers, and drivenby the second driving motor.

Each of the driving gears and each of the image bearing bodiespreferably include at their corresponding ends, a driving coupler and adriven coupler for complimentary mating with the driving coupler.

The driving and the driven couplers preferably include coupling parts attheir ends provided in a non-circular section and complimentarily matedwith each other; and positioning parts extended from a side of each ofthe coupling parts to a predetermined configuration to determine aposture of engagement of the driving and driven couplers.

The image bearing bodies have a radial displacement in which the radiusof the image bearing body varies, and also have intervals A1, A2, A3 andA4 having a predetermined location of maximum radial displacement, andthe first to fourth image bearing bodies may be mounted to satisfy oneof the following equations:

[Equation Set 2]{2π·1+(α2−α1)}·Ro·(1±0.05)=L 1, (1=0, 1, 2, . . . ), (Ro=Ro 1=Ro2)  {circle around (1)}{2π·m+(α3−α1)}·Ro·(1±0.05)=L 1+L 2, (m=0, 1, 2, . . . ), (Ro=Ro 1=Ro3)  {circle around (3)}{2π·n+(α4−α1)}·Ro·(1±0.05)=L 1+L 2+L 3, (n=0, 1, 2, . . . ), (Ro=Ro 1=Ro4)  {circle around (3)}

-   -   where α1 is the angle, which is measured from the transfer        starting location of the first image bearing body to the center        of the interval A1, in the direction of reverse rotation of the        first image bearing bodies,    -   α2 is the angle, which is measured from the transfer starting        location of the second image bearing body to the center of the        interval A2, in the direction of reverse rotation of the second        image bearing bodies,    -   α3 is the angle, which is measured from the transfer starting        location of the third image bearing body to the center of the        interval A3, in the direction of reverse rotation of the third        image bearing bodies,    -   α4 is the angle, which is measured from the transfer starting        location of the fourth image bearing body to the center of the        interval A4, in the direction of reverse rotation of the fourth        image bearing bodies, and    -   Ro1, Ro2, Ro3 and Ro4 are radii of the first through fourth        image bearing bodies.

The image bearing bodies preferably satisfy L1=L2=L3.

The image bearing bodies preferably satisfy L1=L2=L3, while L1, L2 andL3 are positive integer multiples of Sd.

A driving unit is preferably provided for driving the image bearingbodies and the supporting rollers, and each of the image bearing bodiespreferably have at one end a driven coupler for receiving a drivingforce in engagement with the driving unit.

The driven couplers preferably each include a positioning part fordetermining a position of engagement with respect to the driving unit.The driven couplers may be engaged such that the intervals A1, A2, A3and A4 of the respective image bearing bodies are formed at apredetermined angle with respect to the positioning parts.

The driving unit preferably includes a first driving unit for drivingthe first through fourth image bearing bodies at the same time; and asecond driving unit for driving the supporting rollers independently.

The first driving unit preferably includes a plurality of driving gearsprovided to correspond to the plurality of image bearing bodies,respectively, rotating altogether in association with each other, andeach having a driven coupler at one end to engage with the drivencoupler; and a first driving motor for providing a driving force todrive the plurality of driving gears at the same time.

The first and second image bearing bodies are preferably mounted tosatisfy the first equation of the equations 2-1, and satisfy α1=α2.

The first and third image bearing bodies are preferably mounted tosatisfy the third equation of the equations 2-1, and satisfy α1=α3.

The first and fourth image bearing bodies are preferably mounted tosatisfy the third equation of the equations 2-1, and satisfy α1=α4.

The first to fourth image bearing bodies are preferably mounted tosatisfy all of the equations 2-1, and satisfy α1=α2==α4.

The image bearing bodies have a radial displacement in which the radiusof the image bearing body varies, and also have intervals A1, A2, A3 andA4 having a predetermined locations of maximum radial displacement, andat least two of the first through fourth image bearing bodies aremounted to satisfy one of the following equations:

[Equation Set 3]{2π·1+(α2−α1)}·Ro·(1±0.05)=L 1, (1=0, 1, 2, . . . ), (Ro=Ro 1=Ro2)  {circle around (1)}{2π·m+(α3−α2)}·Ro(1±0.05)=L 2, (m=0, 1, 2, . . . ), (Ro=Ro 2=Ro3)  {circle around (2)}{2π·n+(α4−α3)}·Ro(1±0.05)=L 3, (n=0, 1, 2, . . . ), (Ro=Ro 3=Ro4)  {circle around (3)}

-   -   where α1 is the angle of center of the interval A1, measured        from the transfer starting location of the first image bearing        body in the direction of reverse rotation of the image bearing        bodies,    -   α2 is the angle of center of the interval A2, measured from the        transfer starting location of the second image bearing body in        the direction of reverse rotation of the image bearing bodies,    -   α3 is the angle of center of the interval A3, measured from the        transfer starting location of the third image bearing body in        the direction of reverse rotation of the image bearing bodies,    -   α4 is the angle of center of the interval A4, measured from the        transfer starting location of the fourth image bearing body in        the direction of reverse rotation of the image bearing bodies,        and    -   Ro1, Ro2, Ro3 and Ro4 are radii of the first to fourth image        bearing bodies.

The image bearing bodies preferably satisfy L1=L2=L3.

The image bearing bodies preferably satisfy L1=L2=L3, while L1, L2 andL3 may be positive integral multiples of Sd.

The image bearing bodies are preferably mounted to satisfy all of theequations 2-2, and satisfy α1=α2=α3=α4.

The image bearing bodies each have a radial displacement in which theradius of the image bearing body varies, and an interval A having amaximum radial displacement at a predetermined location, wherein, whenone of the supporting rollers has a radial displacement in which theradius varies, and an interval B having a maximum radial displacement,and with reference to a predetermined (X, Y) coordinate system based onthe center of rotation of the supporting rollers and the image bearingrollers, the image bearing bodies and the supporting rollers are mountedto satisfy one of the following equations:

[Equation Set 3]Rd·θd=(2π·1+θox)·Rox·(1±0.05) (1=1, 2, 3, . . . ), (x=1, 2, 3, . . . ),Rd=z·Rox, (z=2, 3, 4, 5, . . . )  {circle around (1)}Rd·θd=Rox·θox·(1±0.05) Rd=θox, (x=1, 2, 3, . . . )  {circle around (2)}(2π·h+θd)·Rd=Rox·θox·(1±0.05) (h=1, 2, 3, . . . ), (x=1, 2, 3, . . . ),Rox=k·Rd, (k=2, 3, 4, 5, . . . )  {circle around (3)}

-   -   where, θd is the angle of the center of the interval B in the        direction of reverse rotation of the supporting rollers along        the axis +X of the (X, Y) coordinate system,    -   θox is the angle measured from the axis +Y of the (x)th image        bearing body in the running direction of the intermediate        transfer medium to the center of the interval A in the reverse        rotational direction of the image bearing body,    -   Rox is the radius of the (x)th image bearing body, and    -   Rd is the radius of the supporting rollers.

The axis +X is preferably defined to be parallel with reference to therunning direction of the intermediate transfer medium, such that whenthe center of the interval B is positioned on the axis +X, the speedchange of intermediate transfer medium reaches a maximum.

When the radius of the supporting rollers is larger than the radius ofthe image bearing bodies by interger multiples, the integer being 2 ormore, the image bearing bodies and the supporting rollers are preferablyarranged to satisfy the first equation of the Equation Set 3.

When the radius of the image bearing bodies equals the radius of thesupporting rollers, the image bearing bodies and the supporting rollersare preferably arranged to satisfy the second equation of Equation Set3.

When the radius of the image bearing bodies is larger than the radius ofthe supporting rollers by an integer multiple, the integer being 2 ormore, the image bearing bodies and the supporting rollers are preferablyarranged to satisfy the third equation of Equation Set 3.

A first driving unit is preferably provided for driving the imagebearing bodies at the same time, and a second driving unit is preferablyprovided for driving one of the supporting rollers.

The image bearing bodies each preferably include a drum body, a drivencoupler engaged with one end of the drum body to receive a driving forcein connection with the first driving unit.

The drum body of each of the image bearing bodies preferably have a markfor indicating the maximum radial displacement interval A.

The driven coupler of each of the image bearing bodies preferablyinclude a positioning part for determining a position of engagement withthe first driving unit, and the image bearing bodies and the drivencouplers are preferably engaged such that the positioning part ispositioned at a predetermined angle with respect to the mark.

The supporting rollers each preferably include a roller body, and adriven coupler engaged with one end of the roller body to receive adriving force from the second driving unit.

The driven coupler of each of the supporting rollers preferably includea positioning part to determine a position of engagement with respect tothe second driving unit, and the roller body is preferably provided witha mark for indicating the interval B at a predetermined angle withrespect to the positioning part.

The image bearing bodies preferably have identical radii.

The image bearing bodies are preferably arranged to satisfy L1=L2=L3.

The image bearing bodies each have a radial displacement in which theradius of the image bearing body varies, and an interval A having amaximum radial displacement at a predetermined location, wherein when,one of the supporting rollers has a radial displacement in which theradius varies, and an interval B having a maximum radial displacement,and with reference to a predetermined (X, Y) coordinate system based onthe center of rotation of the supporting rollers and the image bearingrollers, the image bearing bodies and the supporting rollers are mountedto satisfy one of the following equations:

[Equation Set 3]Rd·θd=(2π·1+θox)·Rox·(1±0.05)(1=1, 2, 3, . . . ), (x=1, 2, 3, . . . ), Rd=z·Rox, (z=2, 3, 4, 5, . . .)  {circle around (1)}Rd·θd=Rox·θox·(1±0.05) Rd=θox, (x=1, 2, 3, . . . )  {circle around (2)}(2·h+θd)·Rd=Rox·θox·(1±0.05) (h=1, 2, 3, . . . ), (x=1, 2, 3, . . . ),Rox=k·Rd, (k=2, 3, 4, 5, . . . )  {circle around (3)}

-   -   where, θd is the angle of center of the interval B in the        direction of reverse rotation of the supporting rollers along        the axis +X of the (X, Y) coordinate system,    -   θox is the angle measured from the axis +Y of the (x)th image        bearing body in the running direction of the intermediate        transfer medium to the center of the interval A in the reverse        rotational direction of the image bearing body,    -   Rox is the radius of the (x)th image bearing body, and    -   Rd is the radius of the supporting rollers.

The axis +X is preferably defined to be parallel with reference to therunning direction of the intermediate transfer medium, such that whenthe center of the interval B is positioned on the axis +X, the speedchange of intermediate transfer medium reaches a maximum.

When the radius of the supporting rollers is larger than the radius ofthe image bearing bodies by an integer multiple, the integer being 2 ormore, the image bearing bodies and the supporting rollers are preferablyarranged to satisfy the first equation of Equation Set 3.

When the radius of the image bearing bodies equals the radius of thesupporting rollers, the image bearing bodies and the supporting rollersmay be arranged to satisfy the second equation of Equation Set 3.

When the radius of the image bearing bodies is larger than the radius ofthe supporting rollers by an integer multiple, the integer being 2 ormore, the image bearing bodies and the supporting rollers are preferablyarranged to satisfy the third equation of the Equation set 3.

The image bearing bodies are preferably mounted to satisfy L1=L2=L3.

The image bearing bodies preferably satisfy L1=L2=L3, while L1, L2 andL3 are a positive integer multiple of Sd.

A driving unit is preferably provided for driving the image bearingbodies and the supporting rollers, and the image bearing bodies eachpreferably have a driven coupler at one end to receive a driving forcein connection with the driving unit.

The driven couplers each preferably have a positioning part fordetermining a position of engagement with the driving unit, and thedriven couplers are preferably engaged such that the intervals A1, A2,A3 and A4 of the respective image bearing bodies are formed at apredetermined angle with respect to the positioning parts.

The driving unit preferably includes a first driving unit for drivingthe first to fourth image bearing bodies at the same time; and a seconddriving unit for driving the supporting rollers independently.

The first driving unit preferably includes a plurality of driving gearsprovided to correspond to the plurality of image bearing bodies,respectively, rotating altogether in association with each other, andeach having a driven coupler at one end to engage with the drivencoupler; and a first driving motor for providing a driving force todrive the plurality of driving gears at the same time.

The first and second image bearing bodies are preferably mounted tosatisfy the first equation of equations 2-1, and satisfy α1=α2.

The first and third image bearing bodies are preferably mounted tosatisfy the third equation of equations 2-1, and satisfy α1=α3.

The first and fourth image bearing bodies are preferably mounted tosatisfy the third equation of equations 2-1, and satisfy α1=α4.

The first to fourth image bearing bodies are preferably mounted tosatisfy all of the equations 2-1, and satisfy α1=α2=α3=α4.

According to yet another aspect of the present invention, an imageforming apparatus preferably includes a plurality of image bearingbodies arranged in a predetermined sequence, and having a radialdisplacement, and also having an interval A having a maximum radialdisplacement; an intermediate transfer medium running in contact witheach of the plurality of image bearing bodies; and a plurality ofsupporting rollers for guiding the running of the transfer medium whilesupporting the transfer medium, and having a radial displacement inwhich the radius varies, and also having an interval B having a maximumradial displacement, wherein, with reference to a predetermined (X, Y)coordinate system based on the center of rotation of the supportingrollers and the image bearing rollers, the image bearing bodies and thesupporting rollers are mounted to satisfy one of the followingequations:

[Equation Set 3]Rd·θd=(2π·1+θox)·Rox·(1±0.05) (1=1, 2, 3, . . . ), (x=1, 2, 3, . . . ),Rd=z·Rox, (z=2, 3, 4, 5, . . . )  {circle around (1)}Rd·θd=Rox·θox·(1±0.05) Rd=θox, (x=1, 2, 3, . . . )  {circle around (2)}(2π·h+θd)·Rd=Rox·θox·(1±0.05) (h=1, 2, 3, . . . ), (x=1, 2, 3, . . . ),Rox=k·Rd, (k=2, 3, 4, 5, . . . )  {circle around (3)}

-   -   where, θd is the angle of center of the interval B in the        direction of reverse rotation of the supporting rollers along        the axis +X of the (X, Y) coordinate system,    -   θox is the angle measured from the axis +Y of the (x)th image        bearing body in the running direction of the intermediate        transfer medium to the center of the interval A in the reverse        rotational direction of the image bearing body,    -   Rox is the radius of the (x)th image bearing body, and    -   Rd is the radius of the supporting rollers.

The axis +X is preferably defined to be parallel with reference to therunning direction of the intermediate transfer medium, such thatwhen thecenter of the interval B is positioned on the axis +X, the speed changeof intermediate transfer medium reaches a maximum.

When the radius of the supporting rollers is larger than the radius ofthe image bearing bodies by an integer multiple, the integer being 2 ormore, the image bearing bodies and the supporting rollers are preferablyarranged to satisfy the first equation of Equation Set 3.

When the radius of the image bearing bodies equal the radius of thesupporting rollers, the image bearing bodies and the supporting rollersare preferably arranged to satisfy the second equation of Equation Set3.

When the radius of the image bearing bodies is larger than the radius ofthe supporting rollers by an integer multiple, the integer being 2 ormore, the image bearing bodies and the supporting rollers are preferablyarranged to satisfy the third equation of Equation Set 3.

A driving unit is preferably provided for driving the image bearingbodies and the supporting rollers, and the image bearing bodies eachpreferably have a driven coupler at one end to receive a driving forcein connection with the driving unit.

The driven couplers each preferably have a positioning part fordetermining a position of engagement with the driving unit, and thedriven couplers are preferably engaged such that the intervals A1, A2,A3 and A4 of the respective image bearing bodies are formed at apredetermined angle with respect to the positioning parts.

The first driving unit preferably includes a plurality of driving gearsprovided to correspond to the plurality of image bearing bodies,respectively, rotating altogether in association with each other, andeach having a driven coupler at one end to engage with the drivencoupler; and a first driving motor for providing a driving force todrive the plurality of driving gears at the same time.

The second driving unit preferably includes a driving gear provided tocorrespond to the supporting rollers, and having a driving coupler atone end for engagement with the driven coupler which is provided at anend of each of the supporting rollers; and a second driving motor fordriving the driving gear.

BRIEF DESCRIPTION OF THE DRAWINGS

The above aspects and features of the present invention will be moreapparent by describing certain embodiments of the present invention withreference to the accompanying drawings, in which:

FIG. 1 is a schematic view showing the structure of a conventional colorimage forming apparatus;

FIG. 2 is a schematic view illustrating the structure of an imageforming apparatus according to an embodiment of the present invention;

FIG. 3 is a perspective view of a driving unit of FIG. 2;

FIG. 4A is a perspective view showing an image bearing bodydisassembled;

FIG. 4B is a perspective view showing a driving coupler of FIG. 3;

FIG. 4C is a perspective view of a driving roller of FIG. 2;

FIG. 5 is a perspective view of an image bearing body and a drivingcoupler of FIG. 4A according to another embodiment of the presentinvention;

FIG. 6 is a schematic view of a radial displacement caused due tomanufacturing tolerances of the image bearing body and the drivingroller of FIG. 2;

FIG. 7A is a view of the process of monitoring an end of the imagebearing body of FIG. 4A;

FIG. 7B is a view of the process of assembling the image bearing bodiesof FIG. 4A;

FIG. 8A is a schematic structure view of an image forming apparatusaccording to a first embodiment of the present invention;

FIGS. 8B and 8C are graphs provided to illustrate image error caused dueto manufacturing tolerances in the radius of the driving roller in thestate of FIG. 8A;

FIG. 9A is a structure view of a conventional image forming apparatus;

FIG. 9B is a graph of an image error caused due to manufacturingtolerances in the radius of the driving roller in the image formingapparatus shown in FIG. 9A;

FIG. 10A is a schematic structure view of an image forming apparatusaccording to a second embodiment of the present invention;

FIG. 10B is a graph of an image error caused due to manufacturingtolerances in the radius of the driving roller in the state of FIG. 10A;

FIG. 11A is a schematic structure view provided for explanation ofanother conventional printer;

FIG. 11B is a graph of an image error caused due to manufacturingtolerances in the radius of the driving roller in the state of FIG. 11A

FIG. 12A is a schematic structure view showing an image formingapparatus according to a third embodiment of the present invention;

FIG. 12B is a graph of an image error caused due to manufacturingtolerances in the radius of the driving roller in the state of FIG. 12A;

FIG. 13 is a structure view of a main part of a conventional imageforming apparatus;

FIGS. 14A to 14D are schematic views of an image error caused due toimage bearing bodies having radial displacement;

FIG. 14E is a view of an overlay of image errors which are caused due tothe radial displacement of each image bearing body;

FIG. 15A is a schematic structure view of a conventional image formingapparatus in which an image bearing body having a radial displacement isemployed;

FIG. 15B is a graph of an image error caused due to the image bearingbodies of FIG. 15A;

FIG. 16A through 16D are a schematic structure views of an image formingapparatus according to a third embodiment of the present invention;

FIG. 17A is a schematic perspective view of a jig which is provided forsetting an image bearing body according to an embodiment of the presentinvention;

FIG. 17B is a schematic perspective view of another example of a jigwhich is provided for setting an image bearing body according to anembodiment of the present invention;

FIG. 18A is a view showing an example in which the driving roller islarger than the image bearing body;

FIG. 18B is a graph of an image error caused due to the driving rollerand the image bearing body in the state of FIG. 18A;

FIG. 18C is a schematic structure view provided for explanation of animage forming apparatus according to a fourth embodiment of the presentinvention;

FIG. 18D is graph showing an image error caused due to the drivingroller and the image bearing body in the state of FIG. 18C;

FIG. 19A is a view showing an example where the radii of the drivingroller and the image bearing body are equal;

FIG. 19B is a graph showing an image error caused due to the imagebearing body and the driving roller in the state of FIG. 19A;

FIG. 19C is a schematic structure view of an image forming apparatusaccording to a fifth embodiment of the present invention;

FIG. 19D is a graph provided for explanation of an image error causeddue to the driving roller and the image bearing body in the state ofFIG. 19C;

FIG. 20A is a schematic structure view showing an example where theradius of the image bearing body is larger than that of the drivingroller;

FIG. 20B is a graph provided for explanation of an image error causeddue to the driving roller and the image bearing body in the state ofFIG. 20A;

FIG. 20C is a schematic structure view provided for explanation of animage forming apparatus according to an sixth embodiment of the presentinvention;

FIG. 20D is a graph provided for explanation of an image error causeddue to the driving roller and the image bearing body in the state ofFIG. 20C;

FIG. 21A is a schematic structure view provided for explanation of animage forming apparatus according to a seventh embodiment of the presentinvention;

FIG. 21B is a schematic view of an image error caused due to the drivingroller in the state of FIG. 21A;

FIG. 21C is a schematic view of an image error caused due to the imagebearing body in the state of FIG. 21A;

FIG. 21D is a view provided for explanation of an overlay of imageerrors which are caused due to the image bearing body and the drivingroller in the state of FIG. 21A;

FIG. 22A is a structure view provided for explanation of an imageforming apparatus according to an eighth embodiment of the presentinvention;

FIG. 22B is a schematic view of an image error caused due to the drivingroller in the state of FIG. 22A;

FIG. 22C is a schematic view of an image error caused due to the imagebearing body in the state of FIG. 22A;

FIG. 22D is a schematic view of an overlay of image errors which arecaused due to the image bearing body and the driving roller in the stateof FIG. 22A;

FIG. 23A is a structure view of an image forming apparatus according toa ninth embodiment of the present invention;

FIG. 23B is a schematic view of an image error caused due to the drivingroller in the state of FIG. 23A;

FIG. 23C is a schematic view of an image error caused due to the imagebearing body in the state of FIG. 23A; and

FIG. 23D is a schematic view of an overlay of image errors which arecaused due to the image bearing body and the driving roller in the stateof FIG. 23A.

It should be understood that throughout the drawings, like referencenumbers are used to depict like elements features and structure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Certain exemplary embodiments of the present invention will now bedescribed in greater detail with reference to the accompanying drawings.

The matters defined in the description such as a detailed constructionand elements are provided to assist in a comprehensive understanding ofthe invention. Thus, it should be apparent to those of ordinary skill inthe art that various changes and modifications can be made to theexemplary embodiments described herein without departing from the scopeand spirit of the invention. Also, descriptions of well-known functionsor constructions are omitted for clarity and consiceness in describingexemplary embodiments of the present invention.

Referring to FIG. 2, an image forming apparatus according to anembodiment of the present invention includes a plurality of rotaryrollers 30, 40, 50, 60, a belt 70 disposed to run in contact with theplurality of rotary rollers 30, 40, 50, 60, a plurality of supportingrollers 80, 91 to support the belt 70, and a driving unit 100.

In the embodiment illustrated in FIG. 2, the plurality of rotary rollers30, 40, 50, 60 are exemplified as organic photoconductors (OPCs) whichare provided to transfer each of the color images onto the belt 70 in acertain sequence. More specifically, in the following description of anexemplary embodiment, there are four rollers, that is, there are fourOPCs 30, 40, 50, 60 disclosed to independently transfer to the belt 70color images which typically include black (K), cyan (C), magenta (M)and yellow (Y) color images in an overlaying manner.

Additionally, in this particular example of an exemplary embodiment ofthe present invention, the respective OPCs 30, 40, 50, 60 sequentiallytransfer the black, cyan, magenta and yellow color images onto the belt70 as an overlay of a color image along the running direction of thebelt 70 (The OPCs 30, 40, 50, 60 rotate in a clockwise direction in thisexample).

It will be understood that the following described features of anexemplary embodiment of the present invention are equally applicable toother situations such as when there are more or fewer than four OPCs 30,40, 50, 60 employed. One of ordinary skill in the art will alsounderstand that the respective OPCs 30, 40, 50, 60 can be arranged alongthe running direction of the belt 70, in a different sequence from thatwhich is described herein.

Developing units 26, 27, 28, 29 for the respective colors are disposednear, and preferably below the OPCs 30, 40, 50, 60, to form images ineach color on the outer circumference of the OPCs 30, 40, 50, 60. Anysuitable generally-known structures can be applied as the developingunits 26, 27, 28, 29, and one will appreciate that the present inventionis not limited to the structures of the developing units 26, 27, 28, 29described with reference to the exemplary embodiment.

The belt 70 is supported by the driving roller 80, the supporting roller91 and a plurality of T1 rollers 93 to run in a predetermined direction.The belt 70 is run by the rotational force of the driving roller 80 andreceives color images from each of the OPCs 30, 40, 50, 60 in anoverlaying manner. As a result, a full color image is formed and thentransferred onto a printing medium 97 which is passed between the belt70 and the T2 roller 95. The driving roller 80 is connected with thedriving unit 100 and receives the driving force from the driving unit100 to move the belt 70. The supporting roller 91 may be elasticallybiased toward the belt 70 so as to support the belt 70 with a constanttension.

Referring to FIGS. 2 and 3, the driving unit 100 includes a frame 110, afirst driving part 101 provided on the frame 110 that drives the OPCs30, 40, 50, 60, and a second driving part 103 that drives the drivingroller 80.

The frame 110 includes a front frame 111 and a rear frame 112 coupled insubstantially parallel relation with each other.

The first driving part 101 includes a first driving motor 121 installedon the frame 110, first main gears, that is, first through fourthdriving gears 123, 124, 125, 126 for each of the colors corresponding tothe OPCs 30, 40, 50, 60, and first driving couplers 133, 134, 135, 136provided at the rotational axes of the first through fourth drivinggears 123, 124, 125, 126, respectively. The first through fourth drivinggears 123, 124, 125, 126 are arranged at predetermined intervals, anddisposed to rotate between the frames 111 and 112. Reference numeral 122of FIG. 2 denotes a shaft gear which is provided to the driving shaft ofthe driving motor 121 and drives the respective driving gears 124 and125. Additionally, reference numeral 127 denotes an idle gear whichmoves the driving gears 123, 124, 125, 126 in association with eachother. One driving motor 121 is positioned approximately in the middleof the four driving gears 123, 124, 125, 126 to rotate the driving gears123, 124, 125, 126 in the same direction.

The first through fourth driving couplers 133, 134, 135, 136 of eachcolor are rotated together with the driving gears 123, 124, 125, 126.The first through fourth driving couplers 133, 134, 135, 136 areconstructed such that each is engaged with a driven coupler at one endof each OPC 30, 40, 50, 60 to transmit driving force. The drivingcouplers 133, 134, 135, 136 and driven couplers will be described ingreater detail below.

The second driving part 103 includes a second driving motor 141installed on the frame 110, a second main gear, that is, a fifth drivinggear 143 rotatably placed between the frames 111 and 112 and rotated bythe second driving motor 141, and a fifth driving coupler 145 providedat the rotational axis of the fifth driving gear 143. The fifth drivinggear 143 is preferably driven separately from the first through fourthdriving gears 123, 124, 125, 126. The fifth driving coupler 145 isrotated together with the fifth driving gear 143, and is configured suchthat it is complimentarily engaged with the driven coupler at thedriving roller 80 to transmit the driving force. In the exemplaryembodiment, all of the first through fifth driving couplers 133, 134,135, 136 and 145 are shown to have identical coupling structures. Inaddition, other structures of the OPCs 30, 40, 50, 60, and inparticular, the structure of the driven couplers can be understood to besubstantially identical in this embodiment. Accordingly, for the sake ofbrevity, only the K color OPC 30 and the first driving coupler 133 willbe described as a representative example of the others. Similarly,description of the first driven coupler 133 can appropriately replacethat of the fifth driving coupler 145 corresponding to the drivingroller 80.

Meanwhile, being consumables, the OPCs 30, 40, 50, 60 have limitedlifespans that expire when used more than a predetermined number oftimes. The OPCs 30, 40, 50, 60 may be separately installed, or in analternative example, formed integrally with the developing units 26, 27,28, 29 and removably placed in the image forming apparatus body 25.

The OPCs 30, 40, 50, 60 are rotated by the driving force transmittedfrom the driving unit 100. The OPCs 30, 40, 50, 60 are each joined witha passive-driven coupling structure for complimentary mating with thefirst through fourth driving couplers 133, 134, 135, 136 so that theOPCs 30, 40, 50, 60 receive driving force from the driving unit 100 whenmounted in the image forming apparatus body 25. Since the respectiveOPCs 30, 40, 50, 60 can have essentially the same structure, only the Kcolor OPC 30 for forming an image in black (K) as shown in FIG. 4A willbe representatively explained below.

Referring to FIG. 4A, the K color OPC (hereinafter, ‘OPC’) 30 includes acylindrical drum body 31, and a driven coupler 33 engaged with one endof the drum body 31. The drum body 31 is preferably formed of metalmaterial such as stainless steel, and preferably has the configurationof a cylinder with both ends open. A photoconductive layer is preferablycoated or applied to the outer circumference of the drum body 31.

In an exemplary embodiment, one end of the driven coupler 33 ispress-fit to an end of the drum body 31. At the other end, the drivencoupler 33 is provided with a coupling part 33 a with non-circular crosssection, and a positioning part 33 b protruding from a side of thecoupling part 33 a with a predetermined shape. The coupling part 33 a ismated with the first driving coupler 133, shown in FIG. 4B, of thedriving unit 100 to transmit driving force. The positioning part 33 bdetermines the position of the driven coupler 33, that is, determines acoupling angle of the OPC 30 with respect to the first driving coupler133. The same structures as that of the first driven coupler 33 areapplied to the other OPCs 40, 50, 60 and therefore, only the firstdriven coupler 33 of the K color OPC 30 will be representativelyexplained below.

The first driven coupler 33 preferably also includes a shaft 33 cprotruding along the rotational axis of the coupling part 33 a. Theshaft 33 c is coupled to the shaft hole 133 c, shown in FIG. 4B, of thefirst driving coupler 133, to guide the first driving coupler 133 andthe first driven coupler 33 to coaxially mated with each other.

The first driving coupler 133 preferably also includes a coupling part133 a sunken from the end in a non-circular configuration, a positioningpart 133 b sunken from an inner side of the coupling part 133 a, and ashaft hole 133 c. The coupling part 133 a is configured incorrespondence with the coupling part 33 a of the driven coupler 33, andthe positioning part 133 b is configured in correspondence with thepositioning part 33 b of the driven coupler 33.

As shown in FIG. 4C, the driving roller 80 includes a roller body 81,and a driven coupler 83 coupled with an end of the roller body 81. Thedriven coupler 83 may have the same structure as the driven coupler 33of the OPC 30 of FIG. 4A. More specifically, the driven coupler 83includes a coupling part 83 a of a non-circular cross section, and apositioning part 83 b protruding from a side of the coupling part 83 a.The driven coupler 83 of the driving roller 80 is coupled to the fifthdriving coupler 145 as shown in FIG. 3. Because the driven coupler 83has a substantially identical configuration as that of the drivencoupler 33 of FIG. 4A, the fifth driving coupler 145 may haveessentially the same structure as that of the first driving coupler 133which is engaged with the driven coupler 30. Accordingly, descriptionabout the fifth driving coupler 145 can be replaced with the descriptionabout the first driving coupler 133 described with reference to FIG. 4B.

The roller body 81 of the driving roller 80 has a predetermined radiusRd, and a circumference of Sd=2π·Rd. The circumference ‘Sd’ of thedriving roller 80 may be the same or different than the circumference ofthe OPCs 30, 40, 50, 60, and the detailed operation and effect of eachcase will be explained below.

The first through fourth driving couplers 133, 134, 135, 136 and thedriven coupler 33 are preferably configured in an opposing manner. Morespecifically, as shown in FIG. 5, the driven coupler 233 is formed onone end of the roller body 231 of the OPC 230, and the driven coupler233 preferably has a coupling part 234 sunken in a non-circularconfiguration, and a positioning part 235 extending from the couplingpart 234. In this case, the driving coupler 330 corresponding to thedriven coupler 233 of the OPC 230 preferably has, as shown in FIG. 5, acoupling part 331 protruding from an end in a non-circularconfiguration, and a positioning part 332 protruding from a side of thecoupling part 331. The positioning part 332 is complimentarily matedwith the positioning part 235 of the driven coupler 233. The drivencoupler 233 preferably has a shaft hole 236, while the driving coupler330 preferably has a shaft 233 for complimentary mating with the shafthole 236.

In other words, the driven coupler 233 is preferably formed in a concavemanner in the end of the OPC 30, while the end of the driving coupler330 corresponding to the driven coupler 233 is preferably configured ina convex manner. The exemplary embodiment of FIG. 5 is preferablyapplied to all of the OPCs 30, 40, 50, 60, and all of the first throughfourth driving couplers 133, 134, 135, 136.

Although not shown, the structure of the coupler as shown in FIG. 5 canbe applied to the driving roller 80 and the fifth driving coupler 145 aswell. Also, although not described in detail, a variety of couplerstructures can be applied to the supporting roller 91 as well.

Meanwhile, the bodies of the rollers such as OPCs 30, 40, 50, 60 or thedriving roller 80 are typically mass-produced using a metal moldingprocess, as is known in the art. An important aspect in mass-producingroller bodies, that is, in the mass-production of the drum bodies, is toachieve a perfect circle along the outer circumference. However, perfectcircles have yet to be achieved, and there are always tolerances orerrors during processing, which result in the drum body having animperfect cross section. This will now be explained in greater detailwith reference to the organic photoconductor 30 as a representativeexample.

FIG. 6 illustrates in exaggerated fashion, a ‘runout’ occurring in theouter circumference of the drum body 31, which is a radial displacementof the drum body 31 in a predetermined interval. More specifically, therunout causes a radial displacement δo, which is a varyation in theradius according to a rotational cycle of the OPC 30. The radialdisplacement δo can be defined by the maximum and minimum values ofvariations in the nominal radius appearing as a sinusoidal curve in onerotational cycle of the drum body 31, that is, in one rotational cycleof the OPC 30. In this particular embodiment, the maximum radialdisplacement of the roller body 31 is +δo, and the minimum radialdisplacement is −δo. Additionally, as shown in FIG. 6, the interval A isdefined with reference to the maximum radial displacement +δo, whichcontributes to an image error as will be described below. The range ofthe interval A is variable according to the size of the radialdisplacement δo.

As described above, an operator needs to know the interval A in order tocontrol any influence of the roller body 31 cause by radial displacementδo. To this end, as shown in FIG. 4A, the roller body 31 may beindicated with a mark 31 a for indication of the location of interval A.The mark 31 a may be formed on the outer or inner circumference of theroller body 31 to correspond to the center of the interval A. As can beappreciated, the mark 31 a could also be formed at any predeterminedangle away from the center of the interval A, and still identify thelocation of interval A.

As described above, by marking the interval A, the operator can regulatethe relative orientation between the centers of the positioning part 33b of the driven coupler 33 and the interval A during assembly of thedriven coupler 33 with respect to the drum body 31. In other words, asshown in FIGS. 4A and 6, marks 31 a are preferably uniformly formedduring the manufacturing process at a position 45 degrees (+45°) awayfrom the center of the interval A in the rotational direction of theOPC. Then the positioning part 33 b is aligned with the mark 31 a, andthe drum body 31 and the driven coupler 33 are engaged with each other.By assembling all the OPCs in the above-described manner, thepositioning parts 33 b of multiple rollers can be positioned in the samelocation with respect to the interval A.

A variety of methods can be employed to align the positioning part 33 bwith the mark 31 a. For example, as shown in FIG. 7A, the respectiveends of the drum body 31 may be captured through a camera (not shown),and a captured image may be displayed through a screen of apredetermined monitor 200. Reference coordinates (x, y) may be set onthe screen 210, together with a reference complete circle 211. Byplacing such that the reference coordinates (x, y) overlap the referencecomplete circle 211, the center of the interval A of the captured image,that is, the maximum radial displacement +δo can be located.Accordingly, by adjusting such that the maximum radial displacement +δois located 315° in the clockwise direction from the +x axis, the mark 31a is can preferably be indicated on the outer surface, or on the end ofthe drum body 31 at a location which corresponds to the +x axis.

Next, as shown in FIG. 7B, the drum bodies 31 bearing marks 31 a thereonare supported on a first jig 241, with the marks 31 a being located onthe +x axis of the reference (x, y) coordinate system. The drivencoupler 33 is supported on a second jig 242, and in here too, the drivencoupler 33 is arranged such that the positioning part 33 b is positionedon the +x″ axis of the reference coordinate system (x′, y′). In such amanner, the two jigs 241, 242 are oppositely arranged such that thereference coordinate systems (x, y) and (x′, y′) can be aligned witheach other. Then by moving the two jigs 230, 240 towards each other, thedriven couplers 33 are respectively joined with the plurality of drumbodies 31 and therefore, assembled all at the same time with the samerelative orientations. It is possible that the positioning part 33 b ispositioned with respect to the center of the interval A of each drumbody 31. Although it has been described above with reference to oneexample where the plurality of OPCs are assembled at the same time, onewill understand that this should not be considered as limiting, becausethe OPCs can be assembled separately one by one with the same relativeorientations.

In another example, a structure is separately prepared in which the drumbody 31 is rotatably supported on the first jig 241. In this case,separate detecting equipment such as a mark sensor or a monitor detectswhether the mark 31 a is positioned on the +x axis while the drum body31 is rotated. Accordingly, when the mark 31 a is detected to bepositioned on the +x axis, the drum body 31 is stationed, and coupledwith the driven coupler 33 such that the positioning part 33 b isoriented with respect to the mark 31 a.

The OPC 30, after being assembled in the manner as described above, canbe positioned in the center of the interval A, which is −45 degrees fromthe positioning part 33 b. Accordingly, in consideration of the factthat the interval A is located in correspondence with the positioningpart 33 b, it is possible to minimize the generation of image errors dueto deviations in the radius of the OPC drum at interval A by controllingthe positions of the OPCs during assembling and installing.

Although it has been described above that the drum body 31 is assembledwith the driven coupler 33 with reference to the interval A having themaximum radial displacement +δo as an example, it will be appreciatedthat the initial position of the positioning part 33 b can also be setwith reference to other intervals, such as an interval having −δo, orδo=0.

In the manner described above, it should also be understood that theother color OPCs 40, 50, 60 each have an interval A that can be oriented−45 degrees away from the positioning parts 33 b.

The driving roller 80 also has a runout as a result of manufacturingprocesses, and as shown in FIG. 6, has a maximum radial displacement +δdin the circumferential direction. The radial displacement +δd due torunout is, as in the case of the OPC 30, expressed as a sinusoidal curvein a one rotational cycle of the driving roller, and can be defined bythe maximum and minimum values thereof. In this particular embodiment,the maximum radial displacement of the driving roller 80 is +δd, and theminimum radial displacement is −δd. The interval having the maximumradial displacement +δd is an interval B. The interval B also influencesimage errors (described in detail below) due to the driving roller 80,and can be defined in a variety of ways. In the same manner as in theassembling of OPC 30, the driving roller 80 is oriented such that thepositioning part 83 b can be positioned at a predetermined rotationalangle from the center of the interval B by controlling the assembly ofthe roller body 81 and the driven coupler 83 during the assemblingprocess. In the following example, the positioning part 83 a of thedriven coupler 83 is located 45 degrees (that is, −45°) from the centerof the interval B in an opposite direction with respect to the rotationof the driving roller 80.

Meanwhile, the maximum and minimum radial displacements +δd of thedriving roller 80 influences the speed of the belt 70. In other words,the tangential speed maximizes to Vdmax in the center of the interval Bof the driving roller 80. Accordingly, when the interval B and the belt70 contact tightly to the driving roller 80 and therefore, the positionsof force application are aligned with each other, that is, when thedirection of the tangential speed Vdmax runs parallel with the +y axisas shown in an imaginary line of FIG. 6, the belt 70 runs at maximumspeed. Accordingly, as the running speed of the belt 70 changes, colorimages of the OPCs 30, 40, 50, 60 transferred onto the belt 70 may bedistorted so that the images are stretched, blurred or cut. Because thebelt 70 changes speed periodically in a rotational cycle, the imageerror periodically occurs on the belt 70 in correspondence with therotational cycle of the driving roller 80.

Methods according to exemplary embodiments of the present invention toimprove image errors caused due to the radial displacement δd of thedriving roller 80 will be explained below. In the following explanation,it is assumed that the OPCs 30, 40, 50, 60 and the supporting roller 91are free from the runout, and therefore, the radial displacement of theOPCs 30, 40, 50, 60 is δo=0. Accordingly, it is assumed that the radialdisplacement δd of the driving roller 80 is the only influence on theimage being transferred onto the belt 70. As the first step to minimizeimage errors, the influence of the driving roller 80 is minimized andone example of doing so will be described below.

First, as described above with reference to FIG. 6, it is assumed thatthe center of the interval B of the driving roller 80, which has themaximum radial displacement +δd, is positioned on the +45° angle whenthe rotation of the driving roller 80 from the positioning part 83 b isexpressed by the unit of +angles. It should be appreciated that theangle of +45° is a random interval selected for the convenience ofexplanation. The driving roller 80 has a predetermined radius Rd and acircumference Sd=2π·Rd. With the driving roller 80 with the radialdisplacement δd being arranged, the OPCs 30, 40, 50, 60 are arranged tosatisfy one of the following six conditions of equations 1 to reduceimage error of the image being overlain on the belt 70:

[Equation Set 1]L 1=1·Sd·(1±0.05)(1=1, 2, 3, . . . )  (1)L 2=m·Sd·(1±0.05)(m=1, 2, 3, . . . )  (2)L 3=n·Sd·(1±0.05)(n=1, 2, 3, . . . )  (3)L 4=o·Sd·(1±0.05)(o=1, 2, 3, . . . )  (4)L 5=p·Sd·(1±0.05)(p=1, 2, 3, . . . )  (5)L 6=q·Sd·(1±0.05)(q=1, 2, 3, . . . )  (6)

Referring to FIG. 2, the distance between the centers C1, C2 of firstand second driving gears 123, 124 is L1, the distance between thecenters C2, C3 of second and third driving gears 124, 125 is L2, thedistance between the centers C3, C4 of third and fourth driving gears125, 126 is L3, the distance between the centers C1, C3 of first andthird driving gears 123, 125 is L4, the distance between the centers C1,C4 of first and fourth driving gears 123, 126 is L5, and the distancebetween the centers C2, C4 of second and fourth driving gears 124, 126is L6.

Driving couplers 133, 134, 135, 136 are provided to the respectivedriving gears 123, 124, 125, 126, and the OPCs 30, 40, 50, 60 arerotatably engaged with the driving couplers 133, 134, 135, 136.Accordingly, the centers C1, C2, C3, C4 are aligned with the center ofrotation of each of the OPCs 30, 40, 50, 60. Accordingly, it can beunderstood that the OPCs 30, 40, 50, 60 are arranged to satisfy at leastone of the equations 1. Of course, L1 refers to the distance between thecenters C1, C2 of the K color OPC 30 and the C color OPC 40, L2 refersto the distance between the centers C2, C3 of the C color OPC 40 and theM color OPC 50, L3 refers to the distance between the centers C3, C4 ofthe M color OPC 50 and the Y color OPC 60, L4 refers to the distancebetween the centers C1, C3 of the K color OPC 30 and the M color OPC 50,L5 refers to the distance between the centers C1, C4 of the K color OPC30 and the Y color OPC 40, and L6 refers to the distance between thecenters C2, C4 of the C color OPC 40 and the Y color OPC 60.

Accordingly, by setting the image forming apparatus system to satisfy atleast one of the equations in Equation Set 1, image errors due tovariations in the radius of the driving roller 80 can be reduced. FIG. 2shows a preferred embodiment in which the OPCs 30, 40, 50, 60 arearranged to satisfy all the equations in Equation Set 1, such thatL1=L2=L3=Sd.

Image errors occur frequently on the image on the belt 70 due to theinterval B of the driving roller 80 when the OPCs 30, 40, 50, 60 arearranged to satisfy none of the equations 1, and this will be describedin detail below. In the following three examples, the radius Rd of thedriving roller 80 and the radius Ro of the OPCs 30, 40, 50, 60 aremainly considered.

FIG. 8A shows the first example where the driving roller 80 has a biggerradius than the OPC rollers 30, 40, 50, 60 (Rd>Ro). In the example ofFIG. 8A, the OPCs 30, 40, 50, 60 do not satisfy any of the equations inEquation Set 1. In other words, distances between any two of the centersC1, C2, C3, C4 of the respective OPCs 30, 40, 50, 60 are not integermultiples of the circumference Sd of the driving roller 80.

With reference to FIG. 8A, it is assumed that the K color image isinitially transferred with the center of the interval B being positionedat θd (+135°) along the axis +X. As shown in FIG. 8B, the radialdisplacement δd repeats in a sinusoidal manner according to the runningdistance of the belt 70, and the interval B appears once in everyrotational cycle of the driving roller 80. With reference to FIG. 8A, itis assumed that the K image transfer starts from the location F0, andone rotational cycle of the driving roller 80 starts from when thelocation f0 is simultaneously positioned on the axis +x. As shown inFIG. 8B, the center of the image error Rke of the K image caused due tothe interval B, is centered at the location which is distanced apartfrom the location F0 as much as Rd·θd.

When the length of the image transferred by each image bearing body isshort enough, that is, when the location F1 where the K image transferends, and the location F2 where the C image starts transfer, are alignedwith each other, the interval of C image transfer is not influenced bythe interval B. Therefore, no image error due to the interval B does notoccur in the C image. In other words, the K image transfer ends beforethe belt 70 moves by the distance of 1Sd, and the C image transferstarts. Because the location F3 where the C image transfer ends, ispositioned before the belt 70 moves from the location F0 by the distanceof Sd+Rd·θd, no image error occurs in the C image.

Next, when it is assumed that the M image transfer location F4 is thesame as the location F3, an image error Rme occurs in the M image due tothe interval B at the location which is a predetermined distance awayfrom the location F4. Because the distance L4 is not an integer multipleof Sd, the M image transfer start location F4 is positioned after onerotational cycle of the driving roller 80 and the running of the belt 80as much as t1 from the location f1 where the second rotational cycle ofthe driving roller 80 starts. When the respective color image transferintervals are represented in a unit image area P1, the M image error Rmeoccurs at different locations from the starting line SL of the unitimage area P1. In other words, two image errors Rke, Rme appear in theunit image area P1 at a distance apart from one another by Rd·θd−t1.

According to the same principle, the location F5 where the M imagetransfer ends, and the location F6 where the Y image transfer starts,are positioned before the belt 70 is run as much as 2Sd, that is, beforethe driving roller 80 completes a second rotational cycle. Accordingly,an image error Rye due to the interval B also occurs in the Y image.Here, because the Y image transfer start location F6 is positioned apredetermined distance (t2) from the location f2 where the secondrotational cycle of the driving roller 80 starts, the center of the Yimage error Rye occurs away from the starting line SL by t2+Rd·θd.

As explained above, when Rd>Ro, there may be a color image such as a Cimage, which is not influenced by the interval B of the driving roller80 because the OPC rollers complete revolutions more often than thelarger driving roller. Accordingly, a color OPC 40 which corresponds tothe error-free color image, needs not be considered, but at least two ofthe respectively-spaced image errors Rke, Rme, Rye should be alignedwith each other to reduce the frequency of image error occurrence withinthe unit image area P1. In the following description, a method ofaligning the K and M image errors Rke, Rme will be described as oneexample of the present invention.

In order to align the two image errors Rke, Rme, the system is realizedto satisfy the fourth equation of the equation set 1. That is, adistance between the centers C1, C3 of the two OPCs 30, 50 may be set toL4=o·Sd(o=1, 2, 3, . . . ). To this end, as indicated by an imaginaryline in FIG. 8A, the OPC 50 is positioned with its center C3 beingdistanced from the center C1 by oSd with o=1.

Accordingly, as the system is set as described above, if the K imagestarts to be transferred, the center of the K image error Rke occursaway from the starting line SL as much as Rd·θd. The C image is freefrom an image error as mentioned above with reference to FIG. 8B.

Meanwhile, as shown in FIG. 8C, because the center C3 of the M OPC 50 ispositioned at a 1Sd location, that is, at a location f1 where the secondrotational cycle of the driving roller 80 starts, the M image transferstarting location F4 precedes the location F3 where the C image transferends. The location F4 is same as the location f1. Accordingly, thecenter of the M image error Rme due to the interval B occurs away fromthe starting line SL of the unit image area P1 as much as Rd·θd. As aresult, the K and M image errors Rke, Rme are overlapped at the samerelative location. Because the frequency of image error occurrences canbe reduced in a final form of color image being printed, a higherquality print is possible. A Y image error Rye has not been consideredin this description. The Y image error Rye may occur at a differentlocation from the K and M image errors Rke, Rme as described above withreference to FIG. 8B.

Of course, the Y image error Rye can also be aligned with the otherimage errors, by establishing the system to satisfy the fourth and thirdequations of Equation Set 1, or by establishing the system to satisfythe fourth and fifth equations of Equation Set 1. The principleaccording to which K, M, Y image errors Rke, Rme, Rye are overlapped bysatisfying the above conditions, can be easily understood from theabove-mentioned process of aligning the two image errors. Therefore, adetailed description thereof will be omitted for the sake of brevity.

In this particular embodiment, the transfer intervals of each of thecolor OPCs 30, 40, 50, 60 are randomly set, and also the starting andending points of color images are randomly set for the convenience ofexplanation. Accordingly, one will appreciate that the above particularcase should not be construed as limiting. Various examples are possibleunder the condition of Rd>Ro, and the number of image errors of a unitimage area can variously change. It is only clear that, regardless ofthe number of image errors, at least two image errors should alignedwith each other by satisfying at least one of the equations in EquationSet 1 to reduce the total number of occurrences of image errors due tovariations in the radius of the driving roller 80.

Furthermore, although FIG. 8A depicts that the variable o=1 in thefourth equation of the Equation Set 1, this is just one example, and itwill be appreciated that the same effect is obtained when the variable ois an integer number which is equal to or greater than 1.

A second example of the first step will now be described. As shown inFIG. 9A, a first example of image errors is described when none of theequations of Equation Set 1 are met and the driving roller radius isequal to the OPC roller radius (Rd=Ro), followed by the description of amethod of reducing the image error by satisfying at least one of theequations in Equation Set 1.

Referring first to FIG. 9A, it is assumed that the K color image startsto be transferred when +δd, that is, when the center of the interval Bis at a location θd (135°) with reference to the axis +X. As shown inFIG. 9B, an image error Rke due to the interval B occurs away from the Kimage transfer starting line SL by a distance of m1=Rd·θd. That is, thestarting line SL of the K image starts from the location F0, and whenthe starting line SL reaches the location F1 where the K image transferis completed (based on the assumption that one unit image is completedwith one rotational cycle of the driving roller 80), the center of theinterval B of the driving roller 80 is at the initial location (that is,135° of axis X) as shown in FIG. 9A. Here, because the distance L1between the centers C1, C2 of the two OPCs 30, 40 is longer than thecircumference Sd of the driving roller 80 as much as t3, the belt 70moves further from the location F1 by the interval of t3, and the Ccolor image starts transfer from the location F2. It is assumed forpurposes of illustration that the rotational angle of the driving roller80 corresponding to the interval t3 is 45°. In this case, the center ofthe image error Rce of the C image due to the interval B occurs awayfrom the location F1 as much as Rd·θd, but in consideration of the factthat the actual starting line SL of the C image is F2, the center of theimage error Rce occurs away from the starting line SL of the unit imagearea P1 as much as m2=Rd·θd−t3=Rd·(θd−45°)=Rd·90°. Accordingly, thedistance between the centers of the two image errors Rke, Rce are awayfrom the unit image area as much as t3=Rd·90°.

Additionally, because the distance L2 between the centers C2, C3 of theC and M OPCs 40, 50 is longer than Sd as much as t3, the M image startsto be transferred onto the belt 70 when the belt 70 moves from thelocation F3 where the C image transfer ends, by t3=Rd·45°. The locationF4 is at a distance 2t3 from the starting point f2 of the secondrotational cycle of the driving roller 80. Accordingly, when consideringthe fact that the center of the M image error Rme due to the interval Boccurs at a distance that the belt 70 is run from the location f2 byRd·θd, the actual center of the M image error Rme occurs away from thestarting line SL of the M unit image area P1 bym3=Rd·θd−2t3=m1−2Rd·45°=Rd(θd−90°)=Rd·45°. As a result, the M imageerror Rme occurs at a different location of the unit image area P1 fromthe K and C image errors Rke, Rce.

Referring to FIG. 9A, the distance between the centers C3, C4 of the Mand Y OPCs 50, 60 is exemplified as L3=Sd+2t3. In this case, thestarting point B of the third rotational cycle of the driving roller 80precedes the location F5 where the M image transfer ends, by theinterval of 2t3. Because the distance of the two OPCs 50, 60 is furtherdistanced than Sd by the interval of 2t3, the transfer of Y image startsfrom the location F6 where the belt 70 is moved further from thelocation F5 by the interval of 2t3. The location F6 is where the belt 70is moved from the location f3 by the interval of 4t3. Accordingly, animage error due to the interval B does not occur in the third rotationalcycle of the driving roller, and Y image error Rye occurs from alocation which is distanced from the starting location f4 of the fifthrotational cycle by m1=Rd·θd. The Y image error Rye occurs because thelocation f4 is in the Y image transfer interval. In this situation, theY image error Rye occurs at a location F6, which is distanced from thestarting line SL of the unit image area P1 as much asm4=Sd+m1−4t3=Rd·2π+Rd·θd−4Rd·45°=Rd(2π−45°). Because the Y image errorRye occurs in the unit image area P1 at a different relative locationfrom the other image errors Rke, Rce, Rme, it affects the image quality.As described above, when Rd=Ro, at least one image error occurs withineach of the transferred color images. More specifically, although FIGS.9A and 9B depict an example where all of the color images aretransferred within one rotational cycle of the OPCs 30, 40, 50, 60, onecolor image can be completed by the two rotational cycles of the OPCs30, 40, 50, 60. In this case too, one image error occurs in each of thetransferred color images. Accordingly, the image quality can be greatlyimproved by aligning at least two image errors by making at least two ofthe OPC rollers satisfy at least one of the equations in Equation Set 1.

In the following second example of the first step, it will beexemplified that the system is set to satisfy all of the equations ofEquation Set 1. Referring to FIG. 10A, the system is set such thatdistances between the centers C1, C2, C3, C4 of the OPCs 30, 40, 50, 60satisfy L1=L2=L3=Sd. In such a situation, when the center of theinterval B of the driving roller is rotated along the +x axis byθd=135°, it is assumed that the K image is initially transferred ontothe belt 70 at a location F0. If it is assumed that one unit image isformed in one rotational cycle of the OPCs 30, 40, 50, 60, the distanceL1 between the centers C1, C2 of the two OPCs 30, 40 corresponds to aninteger multiple of Sd, that is, L1=1Sd. Accordingly, as shown in FIGS.10A and 10B, the distance from the location F0 where the K imagetransfer starts until the location F1 where the K image transfer ends,corresponds to the distance between the location f0 where the firstrotational cycle of the driving roller 80 begins until the location f1where the second rotational cycle of the driving roller 80 begins.Accordingly, a K image error Rke occurs with its center being located ata distance where the belt 70 is run by m1=Rd·θd.

Next, because the C image transfer start point F2 meets the location F1,and the location f1 also meets the location F1, the center of the Cimage error Rce occurring in the C image due to the interval B ispositioned at the location F1, that is, at a distance from the startingline SL of the C image by m1=Rd·θd. As a result, the K and C imageerrors Rke, Rce are overlapped at the same location.

Furthermore, because L1=L2=1Sd, the M image transfer start point F4 ispositioned at the same location as the C image transfer end location F3,and as the starting point f2 of the third rotational cycle of thedriving roller 80. Because the center of the M image error Rme due tothe interval B is at a m1=Rd·θd distance from the SL location, which isidentical to the location F3, the K and C image errors Rke, Rce can besuperimposed at the same location.

Additionally, because L1=L2=L3=Sd, the Y image transfer start point F6is at the same location as the M image transfer end location F5, and asthe start point f3 of the fourth rotational cycle of the driving roller80. Accordingly, the center of the Y image error Rye due to the intervalB occurs at the location F6, that is, at a distance from the startingline SL of the unit image area P1 as much as m1=Rd·θd. Because the Yimage error Rye occurs at the same location as the overlap of the K, Cand M image errors Rke, Rce, Rme, the number of image errors of unitimage area P1 can be greatly reduced, and as a result, the image qualityimproves.

The third example of the first step where Rd<Ro will now be described.

FIG. 11A shows the system when Ro=2Rd. In the following description, itwill be assumed that each color image is completed by one rotation ofthe OPCs 30, 40, 50, 60. Accordingly, the OPCs 30, 40, 50, 60 rotateonce during two rotations of the driving roller 80 of FIGS. 11A and 11B,and one unit image is formed by one rotation of the OPCs 30, 40, 50, 60.Referring to FIG. 11A, an exemplary system is illustrated in which thecenters C1, C2, C3, C4 of the respective OPCs 30, 40, 50, 60 do notsatisfy any of the equations of Equation Set 1. In such a situation, theK image is initially transferred onto the belt 70 when the maximumradial displacement +δd of the driving roller 80, that is, the center ofthe interval B is rotated from the axis +X by θd (+135°). The locationof the interval B, that is, the location of θd is randomly set for theconvenience of explanation.

When the K image is transferred onto the belt 70 at the K image transferstarting point F0, the starting point f0 of the first rotation of thedriving roller 80 meets the location F0. Accordingly, the center of thefirst K image error Rke1 due to the interval B occurs at a distancewhere the belt 70 is run from the unit image transfer starting line SLas much as m1=Rd·θd. Since the K image transfer is completed by onerotation of the OPC 30, the driving roller 80 rotates twice during thetransfer of the K image. Accordingly, the second K image error Rke2occurs at a distance where the belt 70 is run from the center of thefirst K image error Rke1 as much as 1Sd=2π·Rd. That is, two K imageerrors Rke1, Rke2 occur in one unit image area P1 due to the interval B,and the center of the second K image error Rke2 occurs away from the SLas much as Sd+m1.

Meanwhile, because the distance between the centers C1, C2 of the twoOPCs 30, 40 is L1=1Sd+t3, the C image transfer starts at a location F2where the belt 70 is run by the interval of t3 from the starting pointf1 of the second rotation of the driving roller 80. Accordingly, thecenter of the first C image error Rce1, which is caused due to theinterval B in the second rotation of the driving roller 80, appears at alocation where the belt 70 is moved from the location f1 as much as m1.As a result, the center of the first C image error Rce1 occurs away fromthe starting line SL of the C image transfer by m2=m1−t3. For theconvenience of explanation, the distance corresponding to the intervalt3 is defined to be the length of circumference when the driving roller80 rotates by 45°. Accordingly, m2=Rd·θd−Rd·45°=Rd(θd−45°). As a result,the center of the first C image error Rce1 is at a different locationfrom the centers of the two K image errors Rke1, Rke2 in the unit imagearea P1.

The second C image error Rce2 is caused due to the interval B while theC image is transferred and during the third rotation of the drivingroller 80. The center of the second C image error Rce2 appears at adistance where the belt 80 is moved from the center of the C image errorRce1 by the interval of Sd. Accordingly, the center of the second Cimage error Rce2 is at a distance from the starting line SL of the Cunit image P1 by m2+Sd=Rd(θd−45°)+2π=Rd·(2π+90°).

Because the distance between centers C2 and C3 of the C color OPC 40 andthe M color OPC 50 is L2=Sd+t3, the transfer of the M image starts fromthe point F4. The transfer staring point F4 of the M color image is awayfrom the point F0 by 2Sd+2t3, that is, away from the point f2 by 2t3.Accordingly, the center of the first M color image error Rme1 caused dueto the interval B in the unit image area P1 is positioned away from thepoint f2 by m1=Rd θd. Accordingly, the distance between the center ofthe image error Rme1 and the SL of the unit image area P1 during the Mimage transfer is m3=m1−2t3=Rd·θd−2Rd·45°=Rd (θd−90°). The center of thesecond M color image error Rme2 caused due to the interval B occurs awayfrom the center of the image error Rme1 at a position where the belt 70is moved by a distance of Sd. Accordingly, the second M color imageerror Rme2 occurs away from the SL of the M color image bym3+Sd=Rd·(θd−90°)+2π·Rd=Rd·(2π+45°). Accordingly, the two M color imageerrors Rme1 and Rme2 occur at different locations from the K and C colorimage errors Rke1, Rke2, Rme1 and Rme2.

Referring to FIG. 11A, the distance between the centers C3 and C4 of theM color OPC 50 and the Y color OPC 60 is L3=Sd+2t3. Accordingly, thetransfer starting point F6 of the Y color image is located at a positionwhere the belt 70 is moved from the point f0 by a distance of 3Sd+4t3.Then, the center of the first Y color image error Rye1 caused due to theinterval B during the fourth rotation of the driving roller 8 occursaway from the F6 by a distance of m4. Since the distance between thepoint F6 and the point f3 is 4t3, the distance between the point F6 andthe point f4 is Sd−4t3. Also, the distance between the point f4 and thefirst Y color image error Rye1 is m1. Accordingly,m4=Sd−4t3+m1=2π·Rd−4Rd·45°+Rd·θd=Rd·(2π−45°). The second Y color imageerror Rye1 occurs at a position where the belt 70 is moved from thecenter of the first Y color image Rye1 by a distance of Sd. Accordingly,the center of the second Y color image error Rye1 occurs away from theSL of the Y color unit image P1 by m4+Sd=Rd·(4π−45°). As describedabove, the Y color images Rye1 and Rye1 occur at different positionsfrom the K, C, and M color image errors in the unit image area P1, andas a result, many image errors occur in the unit image area P1.

As described above, the number of image errors can be reduced thief thepair of color image errors occurring in the image area P1 are alignedwith each other. Therefore, the image quality is improved.

Hereinafter, a method of aligning the two C and Y color image errors(Rce1, Rce2)(Rye1, Rye2) with each other will now be described withreference to FIGS. 12A and 12B. To this end, the image forming system isarranged to satisfy the sixth equation of Equation Set 1. That is, thetwo OPCs 40 and 60 are arranged as shown in FIG. 12A to satisfy L6=q Sd.FIG. 12A shows L6=3Sd. That is, the distance between the centers C2 andC4 of the C color and Y color OPCs 40 and 60 is an integer multiple (inthis example, 3) of the outer circumference Sd of the driving roller 80.The remaining distances L1, L2, L3, L4, AND L5 are not necessarilyinteger multiples of the outer circumference Sd. Comparing theconfigurations illustrated in FIGS. 11A and 12A, only the Y color OPC 60of FIG. 12A varies in location. Therefore, the remaining OPCs 30, 40, 50are understood to be arranged at the same position in FIG. 12A as inFIG. 11A.

When the K color image is transferred from the point F0 in the state ofFIG. 12A, the K color image transfer is performed during the tworotational cycles of the driving roller 80. Accordingly, the first andthe second K color image errors (Rke1)(Rke2) due to the B interval occurat the same location as shown in FIGS. 11A and 11B. That is, the centerof the first K color image error (Rke1) occurs away from the pointf0(F0) at a distance where the belt 70 is moved by a distance ofm1=Rd·θd, and the center of the second K color image error (Rke2) occursaway from the point f0 at a distance where the belt 70 is moved bySd+m2.

The first and the second C color image errors (Rce1)(Rce2) occur at thesame location during the transfer of the C color image as shown in FIG.11B. That is, as shown in FIG. 12B, the center of the first C colorimage error Rce1 occurs away from the point F2 (the transfer startingline SL of the C color unit image) at a distance where the belt 70 ismoved by m2=m1−t3=Rd·θd−Rd·45°=Rd(θd−45°). The center of the second Ccolor image error Rce2 occurs away from the point f2 at a distance wherethe belt 70 is moved past F2 by m2+Sd=Rd·(θd−45°)=2π·Rd=Rd·(2π+90°).

The first M color image error (Rme1) occurs away from the point F4 at adistance where the belt 70 is moved by a distance of m3=m1−2t3=Rd θd−2Rd·45°=Rd·(θd−90°). The center of the second M color image error Rme2occurs away from the point F4 at a distance where the belt 70 is movedby a distance of m3+Sd=Rd·(θd−90°)+2π·Rd=Rd·(2π+45°).

Since the center C4 of the Y color OPC 60 is distanced away from thecenter C2 by a distance of 3Sd, the point F6 from which the Y colorimage is transferred is located a distance where the belt 70 is movedfrom the point F2 from which the C color mage is transferred by threetimes the circumference of Sd (3Sd). Then, the point F6 is distancedaway from the point f4 by t3, and the center of the first Y color imageerror Rye1 occurs away from the point f4 by m1 due to the interval B.Accordingly, since the center of the first Y color image error Rye1occurs from the point F6, that is, from the transfer starting line (SL)of the Y color image, at a distance where the belt 70 is moved bym4=m1−t3=Rd·θd−Rd·45°=Rd·(θd−45°)=m2, the two image errors (Rce1, Rye1)are aligned with each other in the unit image area P1. Also, the centerof the second Y color image error Rye2 occurs away from the point F6 ata distance where the belt 70 is moved bym4+Sd=m2+Sd=Rd·(θd−45°)+2π·Rd=Rd·(2π+90°). Accordingly, the second Cimage error (Rce2) and the second Y image error (Rye2) of the unit imagearea P1 are positioned away from the SL by the same distance andtherefore, are aligned with each other.

As described above, in the full color image area P1 where the images aretransferred to the belt 70 through all of the four OPCs 30-60, there aresix image errors including the overlay of image errors (Rke1,Rye1)(Rke2, Rye2), which is 2 errors less than the 8 image errorsoccurring in the construction of FIG. 11A. Accordingly, the imagequality can be improved because the number of image errors decreases andthus, a reliability of a product can be improved.

In the above embodiment explained with reference to FIG. 12A, only thedistance L6 between the centers of the C and Y color OPCs 40 and 60 areinteger multiples of Sd. However, if at least one of the remainingdistances L1, L2, L3, L4 and L5 satisfies the same conditions, thenumber of image errors aligned with each other can increase. Also, ifRo>Rd and the system is arranged to satisfy L1=L2=L3=n·Sd (n=1, 2, 3, .. . ), the 8 image errors decrease to the four overlaying images, andthus, the image quality can be improved. Since such an effect can befully understood by referring to the previous descriptions withreference to FIGS. 12A and 12B, a detailed description will be omittedfor the sake of brevity.

Also, in the description of an exemplary embodiment of the presentinvention using the Equation Set 1, the method of minimizing theoccurrence of image errors caused due to the runout of the drivingroller 80 is highlighted. However, this should not be considered aslimiting. If an image error occurs due to a runout of the supportingroller 91, the occurrence of the image error can be minimized by usingthe above method and Equation Set 1 making any of the lengths L1-L6equal to an integer multiple of the circumference of the supportingroller 91.

That is, the system is arranged to satisfy at least one of the equationsin Equation Set 1 with respect to one of the driving roller 80 or thesupporting roller 91 based on which has a greater radial displacementand greater effects on the image. On the other hand, with respect to theother roller, the runout can be minimized through a post-process such asprecision machining, and the frequency of the image error occurrencecaused by the supporting rollers of the belt 70 can be reduced.

As described above with reference to FIGS. 4A though 7B, each OPC 30,40, 50, 60 has an interval A having the maximum radial displacement +δo.An image error due to the interval A such as image blurring, imagestretch, and image cut, occurs both in the color images formed on therespective OPCs 30, 40, 50, 60 and in the transfer image transferred tothe belt 70. These will now be explained in greater detail withreference to FIG. 13. For example, in order to form a K color image, theOPC 30 is rotated and a surface of the OPC 30 rotated in contact withthe charging roller 35 is charged with a predetermined electricpotential. However, because the radius Ro of the OPC 30 increases asmuch as +δ0 due to the radial displacement +δo in the interval A, theinterval A has a maximum tangential speed Vmax. Because the interval Apasses more rapidly when being rotated in contact with the chargingroller 35, the interval A receives less electric charge and thus has alower electric potential than the other parts of the OPC 30.

Also, an electrostatic latent image is formed by scanning the surface ofthe OPC 30 with laser beams emitted from a laser scanning unit 31.During this procedure of forming the latent image, the interval A hasincreasing tangential speed Vmax. Accordingly, the interval A may have adecreasing amount of laser scan and increasing scan areas. Theelectrostatic latent image formed on the OPC 30 passes through thedeveloping roller 33 so that toner particles are attracted from thedeveloping roller 33 to the electrostatic latent image. Therefore, avisible image is obtained. During this procedure, however, the intervalA passes in contact with the developing roller 33 more rapidly, andalso, the amount of laser energy scanned onto the electrostatic latentimage is less. As a result, less developer is attracted to theelectrostatic image, and the developer attracted to the electrostaticimage blurs due to the faster tangential speed. In summary, the visibleimage obtained on the surface of the OPC 30 through a series ofprocedures such as charging, laser scanning, and developing has an imageerror such as image blurring, image stretch, and image cut, in theinterval A.

As described above, the visible image is formed on the surface of theOPC 30, but has an image error in the interval A prior to beingtransferred to the belt 70. The visible image which is transferred tothe belt 70 may have an image error due to the interval A. As describedabove, the image error occurs due to the interval A caused by the runoutof the OPC 30, 40, 50, 60, independently from the runout of the drivingroller 80 as described above. Because one image error occurs in each OPC30, 40, 50, 60 due to the runout of the OPCs 30, 40, 50, 60, a fullcolor image may have a great number of image errors. For example, ifeach OPC 30, 40, 50, 60 is designed to rotate two times to form a unitimage, two image errors may occur in each OPC 30, 40, 50, 60. As aresult, a full color image has total eight image errors due tovariations in radius of the OPCs 30, 40, 50, 60.

Meanwhile, the center of the interval A, which causes the image error tooccur, can be arranged at a predetermined position with respect to thepositioning part 33 b of the driven coupler 33 as explained withreference to FIGS. 4A through 7B. That is, a mark 31 a is formed on thedrum body 31 at a predetermined angle from the center of the interval A,and the OPCs 30, 40, 50, 60 are assembled by coinciding the mark 31 awith the positioning part 33 b. Also, the OPCs 30, 40, 50, 60 canadvantageously be assembled using a jig without requiring an extrameasurement device.

Hereinafter, an image error occurring when the images are formed byusing the respective OPCs 30, 40, 50, 60, each being mounted to have thecenter of the interval A positioned at a predetermined angle withrespect to the positioning part 33 b, will be described. Additionally, amethod of reducing the number of image errors and an effect thereofaccording to an embodiment of the present invention will be described.

The description will mainly focus on the interval A of the respectiveOPCs 30, 40, 50, 60 which has radial displacement +δo. However, the likedefinition can be applied to an area having −δo. Because the image errorbecomes more problematic in the area having +δo than in the area having−δo, the description will be made mainly about the area having +δo.

For the convenience of the following exemplary explanation, it isassumed that the driving roller 80 and the supporting roller 91 formcomplete circles which have no runouts. It is also assumed thatvariations in speed of the belt 70 due to a radial displacementdifference of the driving roller 80 and the supporting roller 91 doesnot occur, and that the belt 70 runs at a constant speed.

As shown in FIGS. 14A though 14D, the respective color OPCs 30, 40, 50,60 are arranged in a manner so that the positioning parts 33 b of therespective driven couplers are positioned at different angles from oneanother in a counterclockwise direction (+ angle) from the axis Y. Inthis state, starting lines SL of unit images P2 are consecutivelytransferred to contact position Po1, Po2, Po3, and Po4 of the belt 70with respect to the respective OPCs 30, 40, 50, 60, thereby forming anoverlay of color images. The respective OPCs 30, 40, 50, 60 are arrangedto have intervals A1, A2, A3, and A4, each having a maximum radialdisplacement +δo at a predetermined angle from the positioning part 33b. Also, as shown in FIGS. 14A through 14D, it is assumed that therespective OPCs have different postures at the beginning of thetransfer. In the following, as case will be described in which thestarting lines SL of the respective unit images P2 are transferred tothe point Po1, Po2, Po3, and Po4 on the +Y axis of the respective OPCs30, 40, 50, 60. That is, the postures of the respective OPCs 30, 40, 50,60 refers to postures of the respective OPCs 30, 40, 50, 60 at thebeginning of consecutive transfer of the respective color images K, C, Mand Y.

As shown in FIG. 14A, if a printing starts when the K color OPC 30 is incontact with the axis +Y, the center of the interval A1 is positionedwith respect to the +Y axis in a counterclockwise direction by angle ⊖1.In FIGS. 14A through 14D, the centers of the intervals A1˜A4 aredistanced from the positioning part 33 b by 45°. Accordingly, the Kcolor image error Oke occurs at a predetermined position of the unitimage area P2 of the belt 70 due to the interval A1. The image error Okemay include an image blurring, an image stretch, and an image cut. Thatis, because the tangential speed Vmax of the OPC 30 varies more rapidlyin the interval A of the surface of the OPC 30 than the other intervals,a stretched electrostatic latent image is formed on the OPC (on theother hand, since the area Ho has a minimum tangential speed Vmin, ashrunken electrostatic latent image may occur). If the developing roller33 performs a developing in contact with the OPC 30, there occurs animage blurring in the interval A1 due to difference from a nominal speedof the OPCs. When the image is transferred to the belt 70 through theabove processes, there occurs a speed difference between the interval A1and the belt 70 in contact with the interval A1, which results in theimage blurring. The center of the K color image error Oke occurs awayfrom the starting line SL by 1ok=Ro1·⊖1.

As shown in FIG. 14B, the C color OPC 40 has the center of the intervalA2 positioned with respect to the +Y axis such that ⊖2=+135°. In thiscase, the C color image error Oce occurs in the unit image area P2 dueto the interval A2 and at a different position from that of the K colorimage error Oke. That is, the C color image error Oce occurs away fromthe starting line at position 1oc=Ro2·⊖2.

Referring to FIG. 14C, the center of the interval A3 of the M color OPC50 is positioned with respect to the +Y axis in a counterclockwisedirection such that ⊖3=+225°. Accordingly, the M color image error Omeoccurs in the unit image area P2 due to the interval A, and occurs at adifferent position from the K and C color image errors Oke and Oce. Thatis, the M color image error Ome occurs away from the staring line SL atposition 1om=Ro3·⊖3.

Referring to FIG. 14D, the center of the interval A4 of the Y color OPS60 is positioned with respect to the +Y axis in a counterclockwisedirection such that ⊖4=+315°. Accordingly, a Y color image color Oyeoccurs in the unit image area P2 due to the interval A4 of the OPC 60,but occurs at different positions from those of the K, C, M color imageerrors (Oke)(Oce)(Ome). That is, the Y color image error occurs awayfrom the starting line SL at position 1oy−Ro4·⊖4.

Referring to FIGS. 14A and 14D, if Ro1=Ro4=Ro, a distance between thecenters of the image erros Oke and Oce is 1oc−1ok=Ro·(⊖2-⊖1). As aresult, since the color image errors are misaligned in the unit imagearea P2, the respective color image errors Oke, Oce, Ome, Oye arelocated at different positions on the overlay of color images, whichdeteriorates an image quality.

An example of the above will be described in greater detail withreference to FIGS. 15A and 15B.

FIG. 15A shows the OPCs 30, 40, 50, 60 of FIGS. 14A through 14D in asingle view. In FIG. 15A, since the respective OPCs 30, 40, 50, 60 areillustrated with reference to staring points Po1, Po2, Po3, and Po4 fortransferring the respective color image, time is not taken into accountin the following description.

FIG. 15B shows a radial displacement (δok, δoc,δom,δoy) per color in theform of sinusoidal curve according to a rotational cycle of the OPCs 30,40, 50, 60. In the drawing, the intervals between centers of each OPC30, 40, 50, 60 are not considered, and the radial displacement and imageerror per color are shown based on transfer points Po1, Po2, Po3, Po4per color. The radius of each OPC 30, 40, 50, 60 is uniform, that is,Ro1=Ro2=Ro3=Ro4=Ro.

The first graph of FIG. 15B shows a radial displacement (δ ok) accordingto the rotational cycle of the OPC 30, assuming that the transfer of theK color OPC 30 starts in a position shown in FIGS. 14A and 15A. Ifassuming that an K unit image is formed when the OPC 30 is once rotated,the transfer of K color image starts from the point Po1, and therefore,the center of K image error Oke by the interval A1 occurs in onerotational cycle (unit image area; P2). That is, a distance traveled by1ok=Ro·θ1=Ro·45° from the SL point for the belt 70 to transfer the Kcolor image.

The second graph of FIG. 15B shows a radial displacement (δ oc)according to rotational cycle of the C color OPC 40 disposed as shown inFIG. 14B. The transfer of C color starts from the point Po2, and thepoint Po1, in which the transfer of K color image starts, aligns withthe line SL, in which the transfer of C color image starts, at the pointPo2. Referring to FIG. 15A, the interval between two points Po1 and Po2on actual belt 70 is the distance L1 between the rotation centers C1, C2of the two OPCs 30, 40. Accordingly, the SL of K color image startingthe transfer from the point Po1 meets the point Po2 after the belt 70travels L1 distance to start the transfer of C color image. In FIG. 14B,the interval A2 is located in a phase from the point Po2 to θ2 (115°),and therefore, the center of the C color image error Oce is located inthe SL based on one rotational cycle (unit image area; P2). That is, thedistance the belt 70 is traveled is 1oc=Ro·θ2(115°) from the point Po2.If both of the two image errors Oke, Oce are generated in the unit imagearea P2, the graph shows that the interval between the centers of thetwo image errors Oke, Oce is 1oc−1ok=Ro·(θ2−θ1) as explained in FIG.14B.

The third graph of FIG. 15B shows a radial displacement (δ om) accordingto the rotational cycle of the M color OPC 50 disposed as shown in FIG.14C. The center of interval A3 is located on the phase of θ3 (225°) fromthe point Po3 in which the transfer of M color starts. The M color imageerror Ome by the interval A3 is generated in the M color image when theM OPC 50 is rotated by θ3. Accordingly, the center of the M color imageerror Oce is generated in the distance where the belt 70 travels fromthe point Po3 to 1om=Ro·θ3. When one rotational cycle of the OPC 50 isdivided into 360°, the distance 1om=Ro·θ3 lies at a location which isrotated by approximately 225° from the point Po3. Accordingly, thedistance between centers of two image errors Oce, Ome is1om−1oc=Ro·(θ3−θ2). The interval between points Po2, Po3 represents thedistance L2 between the rotational centers C2, C3 of the two OPCs 40,50, and the SL of K color image and C color image aligns at the pointPo3 with SL of M color image superimposed by the OPCs 30, 40.

The fourth graph of FIG. 15B shows a radial displacement (δ oy)according to rotational cycle of the Y color OPC 60 disposed as shown inFIG. 14D. The center of interval A4 is located in a phase of θ4 (315°)from the point Po4. Accordingly, the Y color image error Oye caused byinterval A4 is generated as OPC 60 is rotated by approximately 315° whenone rotational cycle (unit image area;P2) of the OPC 60 is divided into360°. The center of the Y color image error Oye is located in a distancewhere the belt 70 travels from the point Po4 by 1oy=Ro·θ4=Ro·215°. Theinterval between the centers of the two image errors Ome, Oye is1oy−1om=Ro (θ4−θ3). As each color image is superimposed, image errorsare generated in different positions, and therefore, the quality of theoverlapped color images is deteriorated.

As shown in FIG. 15B, the phase of radial displacements(δok,δoc,δom,δoy) per each color are different with respect to onerotational cycle (unit image area; P2). Accordingly, if assuming thatthe unit image area P2 is formed by one rotational cycle of the OPC, theimage errors Oke, Oce, Ome, Oye per each color are generated atdifferent relative positions. Four image errors are generated atdifferent positions in the unit image area P2.

In FIGS. 14A-14D, and 15B, the radius of each OPC 30 to 60 is uniform asRo, and one unit image is formed by one rotation of each OPC 30, 40, 50,60. However, in practice, one unit image may not correlate with onerotation of an OPC. For example, one unit image may be formed by one,one and half, or two rotations according to the size of the radius ofOPCs 30, 40, 50, 60. Accordingly, more than one image error may beintroduced for each OPC per unit image area. Thus, at least four imageerrors, and possibly more, are generated in the unit image area of afull color image, and therefore, image quality is deteriorated.

In addition, in the foregoing description, it was assumed that all ofthe OPC's have the same radius. Of course, as one of ordinary skill inthe art will appreciate, the principles described above could also beapplied if the OPC drums are not all of equal radius. For example,rather than controlling the distance between rotation centers of OPCs,the same effect can be obtained by controlling the distance betweencontact points of the OPCs. That is, each OPC has a contact point on thebelt 70. By controlling the length between contact points of OPCs alongthe belt, image errors caused by radius deviations in the OPCs can beoverlapped, and thereby reduced. In such a scenario, because the radiiare not equal, the length between centers of rotation may notnecessarily be equal to the length along the belt between contactpoints.

If the position of each radial displacement δof OPCs 30 to 60 is alignedbased on the initial transfer point of each OPC 30, 40, 50, 60, that is,if each OPC 30, 40, 50 is adjusted and arranged to generate each imageerror Oce, Ome, Oye shifted by a certain distance, only a singleoverlapped Ote of four color image errors is generated. As shown in thefifth graph of FIG. 15B, the radial displacement δok,δoc,δom,δoy of eachOPCs 30, 40, 50, 60 appears in a regular pattern in the unit rotationalcycle (unit image area; P2). Because each radial displacementδok,δoc,δom,δoy in the unit rotational cycle (unit image area; P2) isaligned, the image errors Oke, Oce, Ome, Oye per each color are alignedin the same phase.

The fifth graph of FIG. 15B shows image errors generated when each offour OPCs 30, 40, 50, 60 are aligned. However, the total number of imageerrors can be greatly reduced even when the image errors of at least twoOPCs are superimposed. Hereinafter, an exemplary method and operationeffect for overlaying image errors of two or more OPCs will be explainedin detail.

FIGS. 14A-14D, and 15B were previously explained based on a sequence oftransfer points of each OPCs. The following exemplary description willexplain the relationship between OPCs based on the engagementrelationship between each of the OPCs and the driven couplerscorresponding to the OPCs at the time of system installation.

Accordingly, description will now be made with reference to FIGS. 16A to16D.

FIG. 16A shows a point when the K color OPC 30 is initially transferred.The transfer start position with respect to the belt 70 is D1, theposition of the K color OPC 30 contacting the belt 70 is E1, and theangle between E1 and the center of the maximum radial displacement areaA1 is a1 (since the present drawing differs from FIGS. 14A to 14D whichare based on the sequential transfer points of each OPCs, general anglesα1˜α4 are used in FIG. 16A). At this time, the contacting point betweenthe C color OPC 40 and the belt 70 is D2, the corresponding position ofthe C color OPC 40 is E2, and the angle between the E2 and the center ofthe maximum radial displacement area A2 of the C color OPC 40 is α2. Theposition on the belt 70 starting the transfer of the M color OPC 50 isD3, the contacting position of the M color OPC 50 is E3, and the anglebetween the E3 and the center of the maximum radial displacement area A3is α3. The contacting point between the Y color OPC 60 and the belt 70is D4, the corresponding position of the Y color OPC 60 is E4, and theangle between the E4 and the center of the maximum radial displacementarea A4 of the Y color OPC 60 is α4. The distance between C1 and C2 isL1, the distance between C2 and C3 is L2, the distance between C3 and C4is L3, and the radii of the OPCs 30, 40, 50, 60 are Ro1, Ro2, Ro3, Ro4.The OPCs may be mounted to meet any one of the following Equation Set 2.

[Equation Set 2]{2π·1+(α2−α1)}·Ro·(1±0.05)=L 1, (1=0, 1, 2, . . . ), (Ro=Ro 1=Ro2)  {circle around (1)}{2π·m+(α3−α1)}·Ro·(1±0.05)=L 1+L 2, (m=0, 1, 2, . . . ), (Ro=Ro 1=Ro3).  {circle around (1)}{2π·n+(α4−α1)}·Ro·(1±0.05)=L 1+L 2+L 3, (n=0, 1, 2, . . . ), (Ro=Ro 1=Ro4)

In FIG. 16A, the K color OPC 30 is initially transferred to the K unitimage Pk. The starting line SL of the K unit image Pk starts from theposition D1 on the belt 70 and contacts the point E1 of the OPC 30. Thecenter of interval A1 is located in the angle of α1 along the +Y axisfrom the point E1. The C color OPC 40 contacts the position D2 on thebelt 70, and the center position of interval A2 is rotated by α2 fromthe contacting position E2 on the OPC 40. Likewise, each center ofintervals A3 and A4 of the M color OPC 50 and the Y color OPC 60 islocated at the angle rotated by α3, α4 from each E3 and E4.

The K color image is transferred to the K unit image Pk in the state ofFIG. 16A, and the center of the K image error Oke caused by the intervalA1 appears away from the starting line SL by b 1=Ro·α1=Ro1·α1. If thecenter of the C color image error Oce generated in the C unit image Pcby the interval A2 of the C OPC 40 is aligned with the center positionof the image error Oke as the center of the K color image error Okereaches the position Po6 contacting the C color OPC 40 and the belt 70as shown in FIG. 16B, the two image errors Oke, Oce are superimposed atthe same position. Here, the following equation meets the aboveconditions.Ro·α1+L 1=Ro·α2, (Ro=Ro 1=Ro 2)  (1)

Because the distance of L1 is within one rotational circumference length2π·Ro of the OPC 30, that is, L1<2π·Ro, the equation (1) may begeneralized as follows:Ro·60 1+L 1=Ro·(α2+2π·1), (1=0, 1, 2, . . . )  (2)

The above equation (2) can be rewritten as follows:{2π·1+(α2−α1)}·Ro=L 1, (1=0, 1, 2, . . . )  (3)

The above equation (3) corresponds to the first equation of Equation Set2. If two OPCs 30, 40 are arranged to satisfy equation (3), the imageerrors Oke, Oce by the intervals A1, A2 of K color and C color OPC 30,40 are superimposed at the same position. More specifically, whensetting each OPCs 30, 40 in the image forming apparatus, the K color OPC30 is preferably first set as the S1 and Ro values are set. Then, if acertain α1 value of the K color OPC 30 is substituted using the firstequation of the equations 2, α2 of the C color OPC 40 can be obtained.Therefore, if the C color OPC 40 is properly arranged along axes X and Ybased on the obtained α2, the image errors Oke, Oce can be superimposed.

Additionally, as shown in FIG. 16A, the center of K image error Okeappears away from the starting line SL as much as b1=Ro·α1=Ro1·α1. Ifthe center of the M image error Ome generated in the M unit image Pm bythe interval A3 of the M color OPC 50 is aligned with the center of theK image error Oke as the center of the K image error Oke reaches thecontacting position Po7 with respect to the belt 70 of the M OPC 50 asshown in FIG. 16C, the two image errors Oke, Ome are superimposed at thesame position. To this end, the two OPCs 30, 50 need to meet thefollowing equation (4).Ro·α1+L 1+L 2=Ro·(α3+2π·m), (m=0, 1, 2, . . . ), (Ro=Ro 1=Ro 3)  (4)

Rewriting the above equation (4), the second equation of the EquationSet 2 can be derived as follows:{2π·m+(α3−α1)}·Ro=L 1+L 2, (m=0, 1, 2, . . . ), (Ro=Ro 1=Ro 3)  (5)

In order to align the K color image error Oke with the Y color imageerror Oye generated in the unit image Py by the interval A4 of the Y OPC60 as shown in FIG. 16D, the center of the interval A4 needs be locatedon the point Po8 when the position D1 of the belt 70 is moved from thepoint Po8 by b1=Ro·α1=Ro1·α1. This condition may be represented in thefollowing equation (6).Ro·α1+L 1+L 2+L 3=Ro·(α4+2π·n), (n=0, 1, 2, . . . ), (Ro=Ro 1=Ro 4)  (6)

The above equation (6) can be rewritten as:{2π·n+(α4−α1)}·Ro=L 1+L 2+L 3, (n=0, 1, 2, . . . ), (Ro=Ro 1=Ro 4)  (7)

Equation (7) rewritten from the equation (6) corresponds to the thirdequation of the Equation Set 2.

The aforementioned conditions of Equation Set 2 can be used to controlthe installation position of OPCs 40, 50, 60 so that image errorsgenerated by one of the OPCs 40, 50, 60 can be superimposed on the Kcolor image error Oke generated by the interval A1 of the K OPC 30.

Additionally, contrary to the Equation Set 2, in order to superimposethe image error generated in at least neighboring OPCs 30, 40, 50, 60,the image forming apparatus needs be set to meet at least one of thefollowing equations.

[Equation Set 3]{2π·1+(α2−α1)}·Ro·(1±0.05)=L 1, (1=0, 1, 2, . . . ), (Ro=Ro 1=Ro2)  {circle around (1)}{2π·m+(α3−α2)}·Ro·(1±0.05)=L 2, (m=0, 1, 2, . . . ), (Ro=Ro 2=Ro3)  {circle around (2)}{2π·n+(α4−α3)}·Ro·(1±0.05)=L 3, (n=0, 1, 2, . . . ), (Ro=Ro 3=Ro4)  {circle around (3)}

-   -   where, the equation {circle around (1)} of the Equation Set 3        corresponds to the first equation of the Equation Set 2. The        equations represent the conditions to align the image errors        Oke, Oce generated by the two neighboring OPCs 30, 40. Of course        the above equations refer to a distance between centers of        rotation of the OPCs. Alternatively, the distance between points        of contact on the belt 70 of each OPC could be controlled with        respect to the orientation of the OPC rollers.

The second equation of the Equation Set 3 represents conditions to alignthe image errors Oce, Ome generated by the interval A2, A3 of the C andM color OPCs 30, 40, and the method of overlapping image errors of thetwo OPCs 30, 40 can be easily comprehended through the aforementionedmethod. In other words, if a2 value of the C color OPC 40 is set withset L2 value and Ro value, a3 value of the M color OPC 50 can beobtained through equation {circle around (2)} of the Equation Set 3.Accordingly, by setting the M color OPC 50 based on a3 value, the twoimage errors Oce, Ome can be superimposed.

Equation {circle around (3)} of the Equation Set 3 represents conditionsto superimpose the image errors Ome, Oye generated by the intervals A3,A4 of the M and Y color OPCs 50, 60. At this time, the image errors Ome,Oye of the two OPCs 50, 60 can be superimposed in the same manner asthat explained in FIGS. 16A-16D which is used to superimpose one of theother image errors Oce, Ome, Oye on the image error Oke by the intervalA1 of the K color OPC 30. In other words, when L3 and Ro are provided asset values, and the M color OPC 50 is first mounted to a certainposition, a3 value of the mounted M color OPC 50 can be obtained. Thena3 is substituted for equation {circle around (3)} of Equation Set 3 toobtain a4 value. Accordingly, when the Y color OPC 60 is set tocorrespond to the obtained a4 value, two image errors Ome, Oye can besuperimposed.

In preferred embodiments of the present invention, the distances betweenthe centers C1, C2, C3, C4 of each OPC 30, 40, 50, 60 are identicallyset as L1=L2=L3. Therefore, the Equation Set 2 can be rewritten asfollows:

[Equation Set 4]{2π·1+(α2−α1)}·Ro·(1±0.05)=L 1, (1=0, 1, 2, . . . ), (Ro=Ro 1=Ro 2)  (1){2π·m+(α3−α1)}·Ro·(1±0.05)=2L 1, (m=, 0, 1, 2, . . . ), (Ro=Ro 1=Ro 2=Ro3)  (2){2π·n+(α4−α1)}·Ro·(1±0.05)=3L 1, (n=0, 1, 2, . . . ), (Ro=Ro 1=Ro 2=Ro3=Ro 4)  (3)

In the same manner as described with reference to FIGS. 4A through 7B,the OPCs 30, 40, 50, 60 are mounted so that the intervals A1, A2, A3 andA4 of the respective OPCs 30, 40, 50, 60 are disposed at the same phasewith respect to the positioning part 33 b of the driven coupler 33. Ofcourse the above equations refer to a distance between centers ofrotation of the OPCs. Alternatively, the distance between points ofcontact on the belt 70 of each OPC could be controlled with respect tothe orientation of the OPC rollers.

In this case, L1, Ro and the maximum radial displacement location on thereference OPC, that is, the location of the interval A1 (α1) areobtained. In this embodiment, the K color OPC 30 is preferably set asthe reference OPC. Additionally, the maximum radial displacementlocations (α2, α3 and α4) of the other OPCS 40, 50, 60 are obtained tosatisfy equations 2 and 3, and the respective OPCs 40, 50, 60 aremounted accordingly. In order for visual confirmation of the radialdisplacement locations, the positioning part 33 b of the driven couplermay be fastened in the factory to have a predetermined interval (forexample, 45°) with respect to the respective radial displacementlocations, as shown in FIG. 4A. Therefore, the initial position of theOPCs 30, 40, 50, 60 can be determined with reference to the positioningpart 33 b.

After the maximum radial displacement locations (Q2, Q3 and Q4) of theother OPCS 40, 50, 60 are determined with respect to the location a1 ofthe reference OPC 30, the OPCs 30, 40, 50, 60 can be mounted in theimage forming apparatus through at least two methods, which will now bedescribed in more detail.

As a first method, as described above, the respective OPCs 30, 40, 50,60 are mounted such that the positioning part 33 b of the driven coupleris positioned at a predetermined angle with respect to the maximumradial displacement +δo. One of the OPCs, namely, OPC 30 is connected tothe driving coupler 133 as shown in FIG. 3. In a state that the OPC 30is mounted, α1 can be detected using a predetermined detecting device.In this state, α2, α3 and α4 of the other OPCs 40, 50, 60 are obtainedby applying L1, L2, L3, Ro and α1 to the second and third equations ofEquations Set 2. The maximum radial displacement +δo of the OPCs 40, 50,60 are controlled using the detecting device to meet the obtained α2, α3and α4, and the OPCs 40, 50, 60 are engaged with the driving couplers134, 135 and 136. According to another example of the first method, theOPCs 30, 40, 50, 60 may be connected to the driving couplers 133-136 ina state that the driving coupler 133 connected to the K color OPC 30,which is the reference OPC, is set to a predetermined position and theother driving couplers 134-136 are positioned corresponding to theobtained α2, α3 and α4. Of course, the positioning parts of the drivingcouplers 133-136 can be adjusted by using detecting equipment.

A second method for setting the OPCs is to use a jig. For example, asshown in FIG. 17A, a jig device 300 may be used, in which a plurality ofreference drums 330, 340, 350, 360 are adjustably mounted in a jig frame310. The reference drums 330, 340, 350, 360 correspond to parts of thecolor OPCs 30, 40, 50, 60 which are mounted in the image formingapparatus. Intervals between the reference drums 330, 340, 350, 360 arepreferably the same as those of the driving couplers 133-136 of thedriving unit 100 (FIG. 3) where the OPCs are mounted. The referencedrums 330, 340, 350, 360 respectively have at the front end thereofdriven couplers 333, 343, 353 and 363 corresponding to the drivencoupler 33 of the OPCs 30, 40, 50, 60. Accordingly, the driven couplers333, 343, 353 and 363 are complimentarily mated with the correspondingdriving couplers 133 to 136, respectively. Also, it will be understoodthat positioning parts of the driven couplers 333, 343, 353, 363 arepositioned at the same phase as the positioning part 33 b of the drivencoupler 33 provided to the substantial OPCs 30, 40, 50, 60, that is, ata predetermined angle with respect to the maximum radial displacement+δo of the OPC.

Meanwhile, the reference drums 330, 340, 350, 360 are rotatably insertedin adjusting holes 313 formed at predetermined intervals on the jigframe 310. In addition, the jig frame 310 has fixing members 320corresponding to the respective reference drums 330, 340, 350, 360. Theplurality of fixing members 320 are fastened to the jig frame 310 in athreaded manner such that one end of each is contacted with thereference drums 330, 340, 350, 360 being inserted in the adjusting holes313. Therefore, as the fixing member 320 is rotated toward the referencedrums 330, 340, 350, 360, the one end is tightly contacted with thereference drums 330, 340, 350, 360, thereby immovably fixing thereference drums 330, 340, 350, 360. On the other hand, as the fixingmember 320 is rotated away from the reference drums 330, 340, 350, 360,the reference drums 330, 340, 350, 360 and the fixing member 320 areseparated from each other so that the reference drums 330, 340, 350, 360are rotatable within the adjusting holes 313.

Using the jig device 300, the positions of the reference drums 330, 340,350, 360, instead of the OPCs 30, 40, 50, 60 mounted in the driving unit100, can be controlled and fixed to satisfy any of the equations inEquation Set 2 or 3.

In order to control reference positions of the reference drums 330, 340,350, 360, the first method described above can be used. In other words,α1 of a predetermined reference drum, for example, the reference drum330 is detected and α2, α3 and α4 of the other reference drums 340, 350,360 are obtained with respect to α1. Based on the obtained α2, α3 andα4, the positions of the respective reference drums 330, 340, 350, 360are controlled and fixed in order using the predetermined detectingdevice.

After the reference positions of the reference drums 330, 340, 350, 360are determined, the jig device 300 and the driving unit 100 areapproached to each other and the driven couplers 333, 343, 353, 363 ofthe position-controlled reference drums 330, 340, 350, 360 are coupledwith the driving couplers 133-136. Since the driven couplers 333, 343,353, 363 are fixed, the driving couplers 133-136 appropriately rotate tobe connected with the driven couplers 333, 343, 353, 363. After thedriving couplers 133-136 are position-controlled and coupled with thedriven couplers 333, 343, 353, 363, the jig device 300 and the drivingunit 10 are moved relative to the reference drums 330, 340, 350, 360along an axial direction of the reference drums 330, 340, 350, 360 so asto separate the driving couplers 133-136 from the driven couplers 333,343, 353, 363. Here, the driving unit 100 may be moved by a dedicatedmoving jig and connected with the jig device 300 or alternatively, thejig device 300 may be moved toward the image forming apparatus, with thedriving unit 100 being mounted to the image forming apparatus, therebycontrolling the driving couplers 133-136 of the driving unit 100.

The driving couplers 133-136, after being connected and separated withrespect to the driven couplers 333, 343, 353, 363, are then coupled withthe driven coupler 33 of the OPCs 30, 40, 50, 60. Since positions of thedriving couplers 133-136 are fixed, the OPCs 30, 40, 50, 60 can be setto satisfy Equations 2 and 3.

Accordingly, by using the jig device 300, assembling time is saved inbulk production and radial displacement control of the OPCs isfacilitated.

FIG. 17B shows a third method of using a jig device 300′. The jig device300′ as shown in FIG. 17B is similar to that shown in FIG. 17A, andtherefore, the like elements are given the same reference numerals inthe following description. However, in the jig device 300′ of FIG. 17B,the reference drums 330, 340, 350, 360 have driven couplers 333, 343,353, 363 at one end, respectively, and also have gears 371, 372, 373,374 at the other end, respectively. Idle gears 375 are arranged betweenthe respective gears 371, 372, 373, 374 such that the respective gears371, 372, 373, 374 are disposed to rotate in associate with each other.The reference drums 330, 340, 350, 360 are arranged at predeterminedintervals. Accordingly, by adjusting the position of one reference drum,the position of the other reference drums can also be adjustedaccordingly. The reference drums are fixed in place when the positionsare adjusted, and the fixing process requires a fixing member 320.Unlike in FIG. 17A, the respective reference drums 330, 340, 350, 360 ofFIG. 17B are connected so that they can move in association with eachother. Therefore, only a single fixing member 320 is required to fix one330 of the reference drums to fix all of the reference drums 330, 340,350, 360. For example, if the first reference drum 330 is fixed by thefixing member 320, the other reference drums 340, 350, 360 areaccordingly fixed due to the gears 371, 372, 373, 374, 375. By using thejig device 300′ as shown in FIG. 17B to obtain, for example, a value α1of one reference drum 330. Then as the position of the correspondingreference drum 330 is adjusted based on the obtained value α1, valuesα2, α3, α4 of the other reference drums 340, 350, 360 are automaticallydetermined, and accordingly, the position of the reference drums 340,350, 360 are also adjusted. As a result, the image bearing bodies can beset easily and efficiently.

When the radii Ro of the respective OPCs 30, 40, 50, 60 are all the sameand the distances between centers of the OPCs 30, 40, 50, 60 are all thesame (L1=L2=L3=2π·Rox (x=1, 2, 3, . . . )), α2, α3 and α4 all have thesame value. Therefore, as shown in FIG. 3, the positioning part of theOPCs 30, 40, 50, 60 fastened to the driving couplers 133-136 can bepositioned by uniformly placing the positioning part of the drivingcouplers 133-136 which correspond to the OPCs 30, 40, 50, 60 along the+X axis. As a result, the image errors of each color, generated by +δoof the OPCs 30, 40, 50, 60, can be controlled to overlap in the samerelative position during the sequential transfer. As described withreference to FIGS. 4A through 7B, the +δo of the OPCs 30, 40, 50, 60 canbe regulated with respect to the positioning part 33 b of the drivencoupler 33, thereby realizing an embodiment of the present invention.

To summarize, i) The OPCs 30, 40, 50, 60 and the driving rollers 80respectively have runouts. In order to advantageously control thefrequency of image errors occurring due to +δo and +δd by the runout,the OPCs and the driving rollers are constructed so that the positioningparts 33 b and 83 b of the driven couplers 33 and 83 are positioned atcertain locations with respect to +δo and +δd.

ii) An embodiment and the effect thereof have been described, with theembodiment satisfying the conditions of the Equation Set 1 so that atleast two of the image errors (Rke, Rce, Rme, Rye) by the runout of atleast one of the driving roller 80 and the supporting roller 91 overlapat the same relative location. Here, the image errors generated in theimage area of the corresponding OPC can be overlapped according torelative sizes of the radius Rd of the driving roller 80 and the radiusRo of the OPC and regardless of the number of image errors generated ina unit image.

iii) Hereinbefore, Equation Set 2 and the Equation Set 3 haverespectively been described as conditions of superimposition of at leasttwo of the image errors (Oke, Oce, Ome, Oye) generated by the runout ofthe OPCs 30, 40, 50, 60, without regard to the runout of the drivingroller 80 and the supporting roller 91. More specifically, in order tosuperimpose any one image error of the other OPCs 40, 50, 60 withrespect to the predetermined OPC 30, the system is set up to satisfy atleast one condition of Equation Set 2 or 3. In order to superimpose theimage errors of at least two neighboring OPCs, the system is set up tosatisfy at least one condition of the Equation Set 3. Equation Sets 2and 3 can be generally applied without considering the relative sizes ofL1, L2 and L3, which are distances between the centers of the OPCs 30,40, 50, 60. When L1=L2=L3, the system is set up, generally, on conditionof Equation Set 4. In other words, since the product is substantiallymanufactured to satisfy L1=L2=L3, considering relations in assembly andsize with other various parts, frequency of the image error occurrencecan be effectively reduced by applying any of Equation Sets 2-4.

As a third step for reducing the frequency of the image erroroccurrence, a method for superimposing the image error by the runout ofat least one of the supporting roller 91 and the driving roller 80corresponding to the image error by the runout of the respective OPCs30, 40, 50, 60 will now be described.

As described in FIGS. 8A and 8B, for example, the image error Rke occursin the image area of the K color OPC 30 caused by the interval B of thedriving roller 80. By the interval B, again, the image errors Rme andRye occur as well in the image areas of other OPCs 50, 60. The imageerror Oke occurs by the interval A1 of the K color OPC 30 as describedin respect to FIGS. 15A and 15B. Also, the image errors Oce, Ome and Oyeoccur by the intervals A1, A2, A3 and A4 of the other OPCs 40, 50, 60.When at least two of the image errors respectively generated by theintervals A1-A4 and B of the OPCs 30, 40, 50, 60 and the driving roller80 are superimposed at the same position on the belt 70, influences ofthe image errors on the final full color image can be reduced.

Referring to FIG. 18A, θd refers to an angle formed by a center of theinterval B defining the radial displacement +δd of the driving roller 80counterclockwise from the +X axis, and that θox refers to an angleformed by the radial displacement +δo of the OPC disposed at the x-thposition in a running direction of the belt 70 counterclockwise from the+Y axis. At least one (Rke, for example) of the image errors Rke, Rce,Rme and Rye, which may be generated by the interval B, can besuperimposed with a corresponding image error (Oke) among the imageerrors Rke, Rce, Rme and Rye of the respective OPCs 30, 40, 50, 60, onthe same position on the belt 70, through the following method. Thedriving roller 80 and the OPC 30 are positioned so that the interval A1of the OPC 30 contacts with the belt 70 at a location where the maximumtangential speed Vmax of the driving roller 80 by the interval B has themost influence on the belt 70, that is, when a direction of the maximumtangential speed Vmax is aligned with the +Y axis as shown by animaginary line in FIG. 18A. Thus, in order to superimpose the imageerrors by at least one of the plurality of OPCs 30, 40, 50, 60, forexample, the OPC 30, with respect to the driving roller 80, the systemis set up to satisfy at least one of the following equations in EquationSet 5.

[Equation Set 5]Rd·θd=(2π·1+θox)·Rox·(1±0.05) (1=1, 2, 3, . . . ), (x=1, 2, 3, . . . )and Rd=z·Rox, (z=2, 3, 4, 5, . . . )  {circle around (1)}Rd·θd=Rox·θox·(1±0.05) Rd=θox, (x=1, 2, 3, . . . )  {circle around (2)}(2π·h+θd)·Rd=Rox·θox·(1±0.05) (h=1, 2, 3, . . . ), (x=1, 2, 3, . . . )and Rox=k·Rd, (k=2, 3, 4, 5, . . . )  {circle around (3)}

That is, if the system is set up to satisfy at least one of theequations from Equation Set 5, the image errors occurring due to theradial displacements of the driving roller 80 and the OPCs 30, 40, 50,60 are superimposed at the same position on the belt 70, andaccordingly, the image error appears only at the superimposed locationof the image.

First, a case wherein Rox<Rd will be described on the assumption thatδo=δd.

According to the first equation of Equation Set 5, the image errors bythe driving roller 80 and the objective OPC can be superimposed when theradius Rd of the driving roller 80 is larger than the radius Rox of theOPC of interest by integer multiples equal to or greater than 2.

This will now be described in greater detail with reference to FIGS. 18Aand 18B. FIG. 18A shows a system which is not set up to satisfy thefirst equation of Equation Set 3. Here, the OPC of interest is the Kcolor OPC 30 being disposed at the first position, and Rd=2Ro1.

When Rd=2Ro1, and the driving roller 80 rotates once, the OPC 30 rotatestwice. It is assumed for purposes of this example that a unit image isformed by two cycles of rotation of the OPC 30. Accordingly, one imageerror caused by the interval B of the driving roller 80 and two imageerrors caused by the interval A1 of the OPC 30 occur within one unitimage area.

Hereinbelow, a method of aligning the image error caused by the intervalB of the driving roller 80 with one of the two image errors caused bythe interval A1 of the OPC 30 by resetting the system to satisfy thefirst equation of Equation Set 5 will be described with reference toFIG. 18A.

More specifically, referring to FIG. 18A, a center of the interval A1 ofthe OPC 30 is set to ⊖ox=⊖o1=315° counterclockwise with respect to the+Y axis, and the center of the interval B is set to ⊖d=315° clockwisewith respect to the +X axis. When transfer of K image begins in thisstate, the center of the image error Rke by the interval B during onerotational cycle of the belt 70 occurs on a location where the belt 70is moved from the starting point ‘Po1’ by a distance equal to Rd·θd. Inother words, the center of the image error Rke, generated in the unitimage by the driving roller 80, is generated at distances of Rd·θd fromSL.

While the driving roller 80 rotates by one cycle, the OPC 30 rotates bytwo cycles. Therefore, two image errors, that is, first and second imageerrors Oke1 and Oke2, casued by the interval A1 are generated within theunit image area.

A center of the first image error Oke1 occurs at a distanceRox·⊖ox=Ro1·⊖o1=Ro1·315° away from SL. Since the second image error Oke2occurs at a distance 2π·Ro1 away from the first image error Oke1, thesecond image error Oke2 is distanced away from SL by2π·Ro+Ro1+⊖o1=(2π+⊖o1)·Ro1.

Therefore, in order to align one of the first and second image errorsOke1 and Oke2 caused by the interval A1 of the OPC 30 with the imageerror Rke caused by the interval B of the driving roller 80 at the sameposition, the distance Rd θd from SL to Rke should be the same as thedistance Rox·θox=Ro1·θol from SL to the first image error Oke1, or asthe distance (2π+θo1)·Ro1 from SL to the second image error Oke2.

To generalize the above conditions, the first equation of Equation Set 5can be obtained as follows.Rd·θd=(2π·+θox)·Rox(1=1, 2, 3, . . . ), (x=1, 2, 3, . . . ) andRd=z·Rox, (z=2, 3, 4, 5, . . . )  {circle around (1)}

By applying Rd=2Ro1 to the first equation of the equations 3, the OPC 30and the driving roller 80 are mounted to satisfy ⊖o1=2·⊖d⁻²π. Because⊖d=315°, ⊖o1=630°−2π=270° is satisfied. That is, when the driving roller80 is initially positioned at a position moved by 315° clockwise on the+X axis with respect to the interval B, according to the first equationof Equation Set 5, the interval A1 of the OPC 30 can be disposed at aposition moved by 270° with respect to the +Y axis as shown in FIG. 18C.Since the interval A1 is pre-positioned, herein, at the phase 45°counterclockwise from the positioning part 33 b of the driven coupler,the interval A1 can be easily adjusted to satisfy ⊖o1=270°.

When the OPC 30 and the driving roller 80 are mounted to satisfy thefirst equation of the equations 3, the image errors Oke2 and Rke aresuperimposed at the same distance, that is, byRd·θd=Rd·315°=(2π+θo1)·Ro1=(2π+270°)·Ro1 away from SL. Therefore, only asingle overlay Te1 of superimposed image errors Oke2 and Rke and anon-superimposed image error Oke1 occur in the K color image.

As described above, when the first OPC 30 and the driving roller 80 arearranged so that the image errors thereof are superimposed, one of theimage errors occurring in the interval A2 of the neighboring OPC 40 canbe superimposed with the image error by the driving roller 80. In otherwords, as shown in FIG. 18C, since Rd=2Ro2 in the C color OPC 40 aswell, θo2=θo1=270° is satisfied according to the first equation ofEquation Set 3. Therefore, when the interval A2 of the C color OPC 40 ispositioned 270° counterclockwise with respect to the +Y axis, the centerof the second image error Oce2 caused by the interval A2 is distancedaway from SL by Rd·θd, as shown in FIG. 18D. Accordingly, the imageerror of the C color OPC 40 can also be superimposed at the sameposition as the superimposed image error Te1.

Since the distance between the neighboring OPCs 30 and 40 is notconsidered herein, the image error Rke of the driving roller 80 and theother image errors Oke1 and Oce1 are not necessarily superimposed. Inorder to superimpose the image errors Oke1 and Oke2 caused by theinterval A1 with the image errors Oce1 and Oce2 caused by the intervalA2, the OPCs 30 and 40 are arranged to satisfy the first equation of theEquation Set 2. To explain it in greater detail, the OPCs 30 and 40 havealready been positioned to satisfy the first equation of Equation Set 3.Therefore, as shown in FIG. 18 c, it can be understood thatθo2=θo1=α2=α1. By applying α2=α1 to the first equation of the EquationSets 2 and 3, L1=2π1·Ro1 can be satisfied. Because 1 is an integer, thedistance L1 between the centers C1 and C2 is set to be an integermultiple of a circumference of the OPC 30. When 1=1, the transfer pointPo2 of the C image of the OPC 40 is positioned where the belt 70 ismoved from the starting point ‘Po1 ’ of the K image by SL. Therefore,the image errors (Oke1, Oke2)(Oce1, Oce2) caused by the intervals A1 andA2 are superimposed at the same position. Since the OPCs 30 and 40 aremounted to satisfy the first equation of Equation Set 5, thesuperimposed image errors Oke2, Oce2 are also superimposed with theimage error Rke caused by the interval B of the driving roller 80.

As can be appreciated from the above description, when the radius of thedriving roller 80 is bigger than the radius of the objective OPC by aninteger multiple equal to or greater than two, one of the plurality ofimage errors generated by the OPC of interest can be superimposed withthe image error caused by the driving roller 80 by setting up the systemto satisfy the first equation of Equation Set 5. In the other pluralityof OPCs as well, one of the image errors caused by the correspondingOPCs can be superimposed with the image error caused by the drivingroller 80 by setting up the system to satisfy the first equation ofEquation Set 5.

When the system is set up to satisfy the first equation of Equation Set2, the image errors generated by the two OPCs 30 and 40 can beoverlapped. Thus, the image errors generated by the OPCs 30 and 40 ofinterest can be reduced.

It has been illustrated that the image error Rke due to the drivingroller 80 and the image error due to the two OPCs 30 and 40 areoverlapped, by way of example of the OPCs 30 and 40. It should beunderstood, however, that if at least one of the plurality of the OPCs30 through 60 can be selected and set to satisfy the first equation ofEquation Set 5, the same effect can be obtained to reduce the imageerror generated due to the selected OPC.

It has been illustrated that the first equation of Equation Set 5 andthe first equation of Equation Sets 2 and 3 can be satisfied at the sametime by way of example. It should be understood that the same effect canbe obtained from the OPC of interest when the respective OPCs 30 through60 satisfy at least one of the equations from Equations Set 2, whilealso satisfying the first equation of Equation Set 5. These scenarioscan be understood from the above description, and will not be describedin detail for the sake of brevity.

Now a scenario in which the size of the driving roller and the OPCrollers are equal (Rd=Rox) will be described. It is assumed, forpurposes of this example, that the radius deviations of the drivingroller and the OPC rollers are equal, that is, δD=δo.

Referring to FIG. 19A, Rd=Rox=Ro1. Therefore, when the driving roller 80rotates one time, the OPC 30 of interest also rotates one time. Supposethat the OPC 30 rotates two times and forms a unit image, two imageerrors Oke1 and Oke2 occur at the K unit image due to the interval A1 ofthe OPC 30 as illustrated in FIG. 19B. Since the interval A1 of the OPC30 is positioned at the phase θo1 in the counterclockwise direction withrespect to the +Y axis, the center of the first image error Oke1 occursaway from the transfer start point Po1 by a distance of Ro1·θo1. Thesecond image error Oke2 occurs away from Po1 by a distance of2π·Ro1+Ro1·θo1. The K unit image has two image errors Rke1 and Rke2generated at a certain location caused by the interval B of the drivingroller 80. Still referring to FIG. 19A, the center of the interval B ofthe driving roller 80 is located at the phase θd in the clockwisedirection with respect to the +X axis. Accordingly, the first imageerror Rke1 caused by the interval B appears away from the point Po1 by adistance of Rd·θd, and the center of the second image error Rke2 appearsaway from the point Po1 by a distance of 2π·Rd+Rd·θd as illustrated inFIG. 19B.

As a result, the distance between the centers of the first image errorsRke1 and Oke1 is d1=Ro1·θo1−Rd·θd. Likewise, the distance between thecenters of the second image errors Rke2 and Oke2 is d1=Ro1·θo1−Rd·θd.

As Rd=Ro, when the OPC 30 and the driving roller 80 are set to satisfythe condition d1=0, that is, the condition Ro1·θo1−Rd·θd, the imageerrors Oke2 and Oke2 of the OPC 30 can be overlapped with the imageerrors Rke1 and the Rke2 of the driving roller 80, respectively. Asillustrated in FIG. 19A, δd=225° and δo1=315°. Hence, when the imageerrors Oke1 and Oke2 of the OPC 30 overlap the image errors Rke1 andRke2 of the driving roller 80, the OPC 30 is set to θd=θo1=225° as shownin FIG. 19C. Since the interval A1 of the OPC 30 is predetermined tolocate at a specific phase away from the positioning part of the drivencoupler, the interval A1 of the OPC 30 is set to locate at 225° awayfrom +Y in the counterclockwise direction as described in the exemplaryembodiments of the present invention. Referring to FIG. 19D, the radialdisplacements δo and δd of the OPC 30 and the driving roller 80 appearon the belt 70 with the same pattern period and can advantageously beoverlapped with each other. Consequently, only the overlapping images ofthe image errors Oke1 and Oke2, and Rke1 and Rke2 occur in the unitimage, to thus reduce the total number of the image errors. That is, theunit image area has only the overlapping error Te3 of Oke1 and Rke1 andthe overlapping error of Te4 of Oke2 and Rke2.

While the OPC 30 is set for the driving roller 80 to satisfy the secondcondition of Equation Set 5, if one of the neighboring OPCs 40, 50, 60is set to satisfy one of the conditions of Equation Set 2 or 3, theimage error due to the OPC of interest can be overlapped with respect tothe overlapping error Te3 and Te4. A detailed description thereof hasbeen provided above in reference to FIGS. 6A through 16, therefore adetailed description will not be repeated here for the sake of brevity.

Now a scenario in which the OPC rollers are larger than the drivingroller (Rd<Rox) will be described. Again, it is assumed for purposes ofthis exemplary description that the radius deviations of the OPCs andthe driving roller are equal, that is, δD=δo. As Rd<Rox, the drivingroller 80 has a shorter rotational cycle than the OPCs 30, 40, 50, 60 inview of the rotational cycle over a unit of time. As a result, the imageerror caused by imperfections in the driving roller 80 occur more oftenthan caused by the OPC in the unit image area. The number of imageerrors can be reduced as a whole by overlapping one of the plurality ofthe image errors of the driving roller 80 in the unit image area, withthe image error of the OPC of interest. To this end, the system is setup to satisfy the third equation of Equation Set 5. An embodiment of thepresent invention exemplifies that the radius Rox of the OPC of interestis two times (c=2) longer than the radius Rd of the driving roller 80.Suppose that the unit image of a certain color is formed by fourrotations of the driving roller 80, and correspondingly two rotations ofthe OPC roller. Hence, the unit image area has six image errors in totalcaused by the driving roller and the particular OPC roller, includingfour image errors caused by the driving roller 80 and two image errorscaused by the OPC.

Referring to FIG. 20A, an example is provided in which the thirdequation of Equation Set 3 is not satisfied. The phase of the center ofthe interval B of the driving roller 80 is positioned at θd=225° withrespect to the +X axis. If the transfer of K initiates, the K unit imageis transferred onto the belt 70 as the driving roller 80 rotates fourtimes as illustrated in FIG. 20B. Four image errors Rke1, Rke2, Rke3,and Rke4 caused by the interval B appear in the unit image area atintervals equal to 2π·Rd. The initial image error Rke1 occurs away fromthe transfer start point Po1 by a distance of Rd·θd=Rd·225°. The otherimage errors Rke2, Rke3, and Rke4 sequentially appear away from thepoint Po1 as much as (2π·h+θd)·Rd (h=1, 2, 3).

The OPC 30 is positioned at θo1=45° away from the +Y axis in thecounterclockwise direction. To transfer the K unit image area, the OPC30 rotates two times. Thus, two image errors Oke1 and Oke2 due to theinterval A1 occur with the distance 2π·Ro1 therebetween in the K unitimage area as illustrated in FIG. 20B. The first image error Oke1 occursat a position where the belt 70 is moved from the transfer start pointPo1 by a distance of Ro1·θo1=Ro1·45°. Therefore, the distance betweenthe first image errors Oke1 and Rke1 isd2=Ro1·(0.5θd−θo1)=Ro1·(112.5°−45°)=Ro1·67.5°.

As a result, to overlap the two image errors Oke1 and Rke1, the distancefrom Po1 to the image errors Oke1 and Rke1 should be identical. Whilethe driving roller 80 is set in advance, it is required to satisfy thecondition Rd·θd=Ro1·(45°+67.5°). The third equation of Equation Set 5can be satisfied by setting the OPC 30 to position at 112.5° from +Y ofthe interval A1 in the counterclockwise direction, as illustrated inFIG. 20C. In FIG. 20D, the centers of the two image errors Oke1 and Rke1are at the same location, that is, the centers are aligned away from Po1by a distance of Rd·δd=Ro1·θo1=Ro1·112.5°. The second image error Oke2of the interval A1 overlaps with the third image error Rke3 of theinterval B. Therefore, the six image errors in the K unit image area arereduced to four image errors, to thus reduce the effect of the imageerrors.

The image errors caused by other OPCs can be further reduced by settingthe other OPCs to be in phase with the first OPC. That is, the OPC ofinterest can be set to satisfy at least one of the conditions ofEquation Sets 2-4 while also satisfying the conditions of Equation Set5. In this situation, at least one of the image errors due to thedriving roller 80 can overlap with the image error of the OPC, and atthe same time, the image errors occurring due to the OPCs can overlapwith each other. Therefore, the number of the image errors can bereduced still more.

The following is an explanation as to how to overlap the image error ofthe driving error 80 with at least one of the image errors of at leasttwo OPCs at the same location within the unit image. In this exemplaryembodiment of the present invention, it is exemplified that all theimage errors of the plurality of the OPCs 30, 40, 50, 60 are overlapped,and that the overlapping error of the OPCs 30, 40, 50, 60 is alsoadvantageously overlapped with the image error of the driving roller 80.Note that the radiuses of the OPCs 30, 40, 50, 60 in this example areidentical.

Firstly, referring to FIG. 21A, the radius Rd of the driving roller 80is greater than the radius Rox of the OPCs 30, 40, 50, 60. The radius ofthe respective OPCs 30, 40, 50, 60 is identical Rox=Ro. Distances L1,L2, L3, L4, L5, and L6 defined among the centers C1-C4 of the OPCS 30,40, 50, 60 are set to satisfy all of the conditions of Equation Set 1.That is, the positions of the OPCs 30, 40, 50, 60 are set to satisfyL1=L2=L3=Sd(2π·Rd). Under these conditions, the image error due to theinterval B of the driving roller 80 appears at the same location as theoverlapping unit image that is transferred onto the belt 70 by therespective OPCs 30, 40, 50, 60. If the unit image area is formed duringone rotation of the driving roller 80, the center of color image errorsRke, Rce, Rme, Rye caused by the interval B is distanced away from thestarting line SL by a distance of Rd·θd(315°) in the unit image area, asillustrated in FIG. 21B. Since L1=L2=L3=Sd, the full color unit imagearea which passes through the OPCs 30, 40, 50, 60 and is overlapped onthe belt 70, has a single overlapping error Rte from the image errorRke, Rce, Rme, Rye. Ultimately, by configuring the system to satisfy allof the conditions of Equation Set 1, the number of image errors due tothe interval B of the driving roller 80 can be reduced to one fourth ofthe number of errors of Equation Set 1 is not followed, and the imagequality can thereby be enhanced.

As discussed previously, the respective OPCs 30, 40, 50, 60 have radialdisplacement due to runout. Intervals A1, A2, A3, and A4 of the OPCs 30,40, 50, 60 cause image errors. To overlap the image errors due theintervals A1, A2, A3, and A4, as shown in FIG. 21A, is the OPC's are setto satisfy the conditions of Equations 2-1 and 2-2. In short, L1=L2=L3is satisfied and the radii of the OPCs 30, 40, 50, 60 are identical. Themaximum radial displacement +δ0 of the OPCs 30, 40, 50, 60 is positionedat the same phase 270° from the +Y axis in the counterclockwisedirection. As constructed above, when the driving roller 80 rotates onetime, the OPCs 30, 40, 50, 60 rotate two times to create the unit imagearea.

Two image errors occur from each of the respective OPCs 30, 40, 50, 60,and the centers of the first image errors Oke1, Oce1, Ome1, Oye1 causedby the OPCs 30, 40, 50, 60 are positioned away from the starting line SLby a distance of Ro·270°, as illustrated in FIG. 21C. The centers of thesecond image errors Oke2, Oce2, Ome2, Oye2 caused by the OPCs 30, 40,50, 60 are positioned away from the starting line SL by a distance ofRo·(2π+270°). Originally, the full color overlapping image, which passedthrough the OPCs 30, 40, 50, 60, has 8 image errors Oke1, Oce1, Ome1,Oye1 and Oke2, Oce2, Ome2, Oye2. When the system is set to satisfy theconditions of Equation Sets 2-4, the overlapping error Ote1 of the firstimage errors Oke1, Oce1, Ome1, Oye1 and the overlapping error Ote2 ofthe second image errors Oke2, Oce2, Ome2, Oye2 only appear. Thus, thenumber of the image errors can be reduced to one fourth of the number oferrors caused when the relative locations of the OPC's are notcontrolled according to Equation Set 2.

The overlapping errors Ote1 and Ote2 caused by the OPCs 30, 40, 50, 60may appear at different positions from the overlapping error Rte due tothe driving roller 80 as mentioned above. When the overlapping error Rteis overlapped with one of the overlapping errors Ote1 and Ote2 by theOPCs 30, 40, 50, 60, the number of the image errors in the unit imagearea can be reduced further. To this end, it is set to satisfy the firstequation of Equation Set 5 in FIG. 21A. That is,Rd=2Rox=2Ro1=2Ro2=2Ro3=2Ro4. Note that θox==θo1=θo2=θo3=θo4=270°.Accordingly, the distance Rd−d between SL and the center of Rte is thesame as the distance Ro·(2π+270°) between SL and the center of Ote2. Inother words, as Rd=2Ro and Rd·θd=2Ro·315°=Ro·(2π+270°), the twooverlapping errors Ote2 and Rte are overlapped away from SL by adistance of Rd·θd.

As for the scenario when Rd=2Ro, when the system is set to satisfy thefirst equation of Equation Set 5 in addition to all of the conditions ofEquation Set 1 and Equation Sets 2-4, the unit image area has only thefirst overlapping error Ote1 caused by the OPCs 30, 40, 50, 60 and onlythe overlapping error Te5 of the second overlapping error Ote2 caused bythe OPCs 30, 40, 50, 60 and the overlapping error Rte caused by thedriving roller 80, as illustrated in FIG. 21D. Ultimately, twelve imageerrors including four image errors by the driving roller 80 and eightimage errors by the OPCs 30 through 60 can be reduced to two imageerrors in the unit image area. Thus the total number of image errors isreduced considerably and the image quality is enhanced.

When Rd=Rox, Equation Set 1, Equation Sets 2-4, and Equation Set 5 areall satisfied. In this case, the radii of the OPCs 30, 40, 50, 60 areidentical Rox=Ro. Referring to FIG. 22A, the interval B of the drivingroller 80 is positioned at the phase δd away from the +X axis in theclockwise direction. In an exemplary embodiment of the presentinvention, θd=315°. The distance between the centers C1, C2, C3 and C4of the OPCs 30, 40, 50, 60 is L1=L2=L3=2π·Rd=Sd. The distance betweenthe centers of the OPCs 30, 40, 50, 60 is set to equal an integermultiple of the circumference of the driving roller 80. Such a systemcan satisfy Equation Set 1. To facilitate the understanding ofembodiments of the present invention, suppose that the unit image areais formed by one rotation of the driving roller 80, the interval Baffects the same phase of the respective OPCs 30, 40, 50, 60. Morespecifically, the driving roller 80 of FIG. 22A needs to rotate fourtimes to transfer the overlapped color images of the OPCs 30, 40, 50, 60and form the final full color image. As illustrated in FIG. 22B, thecenters of the image errors Rke, Rce, Rme, Rye occurring within thecolor overlapping image and caused by the interval B are aligned awayfrom the starting line SL by a distance of Rd·θd=Rd·315°. As the OPCs30, 40, 50, 60 are set to satisfy Equation Set 1, all of the imageerrors Rke, Rce, Rme, Rye caused by the interval B appear at the samelocation during the image transfer by the OPCs 30, 40, 50, 60,regardless of the number of the rotations of the driving roller 80.Consequently, only one overlapping image error Rte occurs.

The OPCs 30, 40, 50, 60 are installed to satisfy the condition ofEquation Set 2. In more detail, the OPCs 30, 40, 50, 60 are installedsuch that the intervals A1, A2, A3, and A4 of the respective OPCs 30,40, 50, 60 are positioned at the same angle from the transfer startpoint Po1, Po2, Po3, and Po4 in the counterclockwise direction, that is,at the phase α1=α2=α3=α4. For further understanding of an exemplaryembodiment of the present invention, α1−α4 are set to 315° and it isassumed that one rotation of the OPCs 30, 40, 50, 60 creates the colorunit images. Referring to FIG. 22C, the centers of the image errors Oke,Oce, Ome, Oye due to the intervals A1, A2, A3, and A4 of the OPCs 30,40, 50, 60 appear away from the starting line SL by a distance ofRo·α1=Ro·315° in the unit image area. Therefore, the full color unitimage area generated by the OPCs 30, 40, 50, 60 and transferred onto thebelt 70 in an overlapping manner, has only the overlapping image errorOte of the image errors Oke, Oce, Ome, Oye. The overlapping image Ote isdistanced from the SL by Ro·315°.

The system of FIG. 22A is set to satisfy the second condition ofEquation Set 3. That is, Rd=Rox, and θo1(=α1), θo2 (=α2), θo3 (=α3), andθo4 (=α4) of the respective OPCs 30, 40, 50, 60 equal to 315°, which isalso equal to θd.

Accordingly, the image errors due to the interval B of the drivingroller 80 overlap with the image errors due to the intervals A1, A2, A3,and A4 of the respective OPCs 30, 40, 50, 60. Meanwhile, as shown inFIG. 22B, the image errors Rke, Rce, Rme, Rye due to the driving roller80 overlap as the overlapping error Rte at the same position. The imageerrors Oke, Oce, Ome, Oye due to the OPCs 30, 40, 50, 60 also overlap asthe overlapping error Ote at the same position. In this regard, thesystem is implemented to further satisfying the second condition ofEquation Set 5 so that the two overlapping errors Rte and Ote canadvantageously overlap at the same position. As a result, as illustratedin FIG. 22D, the unit image area has only the final overlapping errorTe6 produced from the two overlapping errors Rte and Ote. The center ofthe final overlapping error Te6 is distanced away from SL byRd·θd=Ro·θo1=Ro·α1. Under the condition of Rd=Ro, when the secondcondition of Equation Set 1, Equation Sets 2-4, and Equation Set 5 aresatisfied at the same time, even more image errors in the unit imagearea can be overlapped. Hence, the number of the image errors decreases,the image quality can be enhanced, and the reliability of the productcan be elevated.

Thirdly, referring to FIG. 23A, a scenario when the OPC rollers arebigger than the driving roller is described, that is, when Rox=h·Rd(h=2,4, 6 . . . ). To ease the understanding of embodiments of the presentinvention, it is exemplified that the radius Rd of the driving roller 80is two times longer than the radius Rox of the OPC. The number of theOPCs of interest is four, and the radii of the respective OPCs 30, 40,50, 60 is Rox=Ro1=Ro2=Ro3=Ro4.

The distance between the centers C1, C2, C3, and C4 of the OPCs 30, 40,50, 60 is L1=L2=L3=2×2π·Rd=2Sd. Accordingly, when the driving roller 80rotates two times, the respective OPCs 30, 40, 50, 60 make one rotation.Suppose that two rotations of the driving roller 80 create the unitimage of a certain color, the interval B of the driving roller 80produces two effects on the color unit image areas, as illustrated inFIG. 23B. Since the distance between the centers of the OPCs 30, 40, 50,60 is an integer multiple of the circumference of the driving roller 80,the centers of the first image errors Rke1, Rce1, Rme1, Rye1 occurringat each color are aligned away from the starting line SL by a distanceof Rd·θd. The centers of the second image errors Rke2, Rce2, Rme2, Rye2are distanced from the starting line SL by 2π·Rd+Rd·θd. As θd=180°, thefirst image errors Rke1, Rce1, Rme1, Rye1 appear away from the SL by adistance of Rd·180°, as the single overlapping image Rte1. The secondimage errors Rke2, Rce2, Rme2, Rye2 appear as the single overlappingimage error Rte2 positioned away from SL by a distance of2π·Rd+Rd·180°=Rd·(2π+180°).

The distances between the centers C1, C2, C3, and C4 of the OPCs 30, 40,50, 60, as defined in the first and second equations of Equation Set 2,are L1=L2=L3=2π·Ro. Since α1=α2=α3=α4=θo1, the centers of the imageerrors Oke Oce, Ome, Oye due to the intervals A1, A2, A3, and A4 of theOPCs 30, 40, 50, 60 are distanced from SL by Ro·θo1 in the unit imagearea, as illustrated in FIG. 23C. Note that in this example θo1=α1=270°.Accordingly, the image errors Oke, Oce, Ome, Oye due to the OPCs 30, 40,50, 60 appear as the single overlapping error Ote positioned away fromSL by a distance of Ro·270°.

The system of FIG. 23A is set to satisfy the third equation of EquationSet 5. Since 2Rd=Rox, which is applied to the third equation of EquationSet 5, (2π·θd)·Rd=Rox·θox=2Rd·θox. Since δd=180°, (2π+180°)·Rd=2Rd·θox.It can be seen that the above equation can be rearranged to(2π+180°)=θox and θox=270°. Since θo1=θo2=θo3=θo4=270°, the respectiveOPCs 30, 40, 50, 60, as shown in FIG. 23A, satisfy the third equation ofEquation Set 5. After satisfying the third equation of Equation Set 5,as aforementioned in reference to FIGS. 23B and 23C, one of the twooverlapping errors Rte1 and Rte2 can be overlapped with the finaloverlapping error Te7 that overlaps with the other overlapping error Oteat the same position as illustrated in FIG. 23D. Because the center ofthe overlapping image error Ote due to the OPCs 30, 40, 50, 60 ispositioned away from SL by a distance of Ro·θo1=(2π+θd)·Rd, the finaloverlapping error Te7 appears away from SL by a distance ofRo·θo1=(2π+δd)·Rd. Ultimately, the final full color unit image has onlythe overlapping error Rte1 and the final overlapping error Te7 asillustrated in FIG. 23D.

In the exemplary embodiments of the present invention, it has beendescribed that the image errors mainly result from the runouts of theOPCs 30, 40, 50, 60, which substantially transfer the image onto thebelt 70, and the runout of the driving roller 80, which drives the belt70, without consideration of the effect on the supporting roller 91.Thus, the effect of the supporting roller 91 is not explained herein. Itis to be understood that the driving roller 80 and the supporting roller(driven roller) 91 are interchangeable in the relationship.

Also, as indicated in the above equation sets, one will understand thatalmost the same effect can be obtained within an error range of ±0.05.

As set forth above, the roller and the roller manufacturing methodaccording to embodiments of the present invention are assembled suchthat the positioning part of the driven coupler is positioned at acertain angle in relation with the maximum radial displacement of theroller body (drum body). Hence, it is possible to control the effect onthe radial displacement of the roller.

The driving unit of the image forming apparatus according to embodimentsof the present invention can control the alignment of the OPCs based ona criterion, to thus control the effect from radius deviations of thebelt supporting roller and/or effects from the radius deviations of theOPCs.

Furthermore, the image forming apparatus controls the locations of theimage errors occurring due to runout of the driving roller and reducesthe number of image errors in the final overlapping image. Therefore,the image quality can be enhanced and the reliability of the product isincreased.

In particular, the frequency of image errors due to runouts of thedriving roller and/or the OPCs can be decreased even withoutconsideration of the radii of the OPCs and the driving roller.

The foregoing embodiments and advantages are merely exemplary and arenot to be construed as limiting the present invention. The presentteaching can be readily applied to other types of apparatuses. Also, thedescription of the embodiments of the present invention is intended tobe illustrative, and not to limit the scope of the claims, and manyalternatives, modifications, and variations will be apparent to thoseskilled in the art.

1. An image forming apparatus, comprising: at least two image bearingbodies arranged in a predetermined sequence, each image bearing bodyhaving a respective point of contact with an intermediate transfermedium; wherein each of the at least two image bearing bodies has aradius deviation, and the at least two image bearing bodies are eachoriented relative to the other such that the radius deviation of oneimage bearing body is in substantial registration along the intermediatetransfer medium with the radius deviation of the other image bearingbody.
 2. The image forming apparatus of claim 1, wherein theintermediate transfer medium is an intermediate transfer belt having atleast two supporting rollers.
 3. The image forming apparatus of claim 1,wherein the intermediate transfer medium is an intermediate transferdrum.
 4. The image forming apparatus of claim 1, wherein the radiusdeviations comprise maximum radius deviations of the respective imagebearing bodies.
 5. The image forming apparatus of claim 2, wherein atleast one of the supporting rollers has a radius deviation, and the atleast two image bearing bodies are configured such that either: (a) adistance between respective points of contact is an integer multiple ofa circumference of the supporting roller; or (b) the circumference ofthe supporting roller is an integer multiple of the distance betweenrespective points of contact.
 6. The image forming apparatus of claim 1,wherein each of the at least two image bearing bodies is engaged with adriven coupler that is engaged in a predetermined position with respectto the respective radius deviation of the image bearing body, and drivencoupler adapted for complimentary mating with a driving coupler thatreceives a driving force.
 7. The image forming apparatus of claim 6,wherein the driven coupler comprises a coupling part having anon-circular shape, and the driving coupler has a complimentarynon-circular shape.
 8. The image forming apparatus of claim 7, whereinthe coupling part comprises a position determining part.
 9. The imageforming apparatus of claim 2, wherein at least one of the supportingrollers has a radius deviation, and at least one of the image bearingbodies is oriented relative to the at least one supporting roller suchthat the radius deviation of the image bearing body is in substantialregistration with the radius deviation of the at least one supportingroller.
 10. A method of manufacturing an image forming apparatus havingat least two image bearing bodies, each of the at least two imagebearing bodies having a respective point of contact with an intermediatetransfer medium and a respective radius deviation, the method comprisingthe steps of: orienting the at least two image bearing bodies such thatthe respective radius deviations are in substantial registration alongthe intermediate transfer medium.
 11. The method of claim 10, whereineach of the at least two image bearing bodies is engaged with a drivencoupler having a predetermined orientation with respect to therespective radius deviation, and the orienting step comprises: orientingthe driven couplers such that the respective radius deviations of the atleast two image bearing bodies are in substantial registration along theintermediate transfer medium.
 12. The method of claim 10, wherein theorienting step further comprises: using a jig to orient the at least twoimage bearing bodies.
 13. The method of claim 11, wherein the orientingstep further comprises: using a jig having at least two engagement partshaving shapes adapted for mating with the driven couplers to orient theat least two image bearing bodies.
 14. A jig for use in manufacturing animage forming apparatus, comprising: at least two engagement parts eachadapted to mate with driven couplers of at least two image bearingbodies to orient the image bearing bodies such that radius deviations ofthe at least two image bearing bodies are in substantial registrationalong an intermediate transfer medium of the image forming apparatus.15. The jig of claim 14, wherein the driven couplers each havenon-circular mating parts, and the at least two engagement parts havecomplimentary non-circular mating parts.
 16. The jig of claim 15,wherein the driven couplers each of positioning parts and the at leasttwo engagement parts mate with the positioning parts to determine theorientation of the respective image bearing bodies.
 17. The jig of claim15, wherein one of the driven coupler and the engagement part isconcave, and the other of the driven coupler and the engagement part isconvex.